fix(gemini): default native maxOutputTokens + strip OpenAI extra_body on Gemini endpoints (#39730)

* fix: respect disabled auto-compaction on context overflow

Port from anomalyco/opencode#30749.

When compression.enabled is false, NO automatic compaction trigger may
fire. The proactive token-threshold paths (preflight + post-response
should_compress gate) already honoured the setting, but the three
provider-overflow recovery paths in the agent loop — long-context-tier
429, 413 payload-too-large, and context-overflow — called
_compress_context() unconditionally, silently compressing and rotating
the session against the user's explicit choice.

Add a single guard at the top of the overflow-recovery dispatch: when
compression is disabled and the error is one of those three overflow
classes, surface a terminal error (compaction_disabled: True) telling the
user to /compress manually, /new, switch to a larger-context model, or
reduce attachments. Manual /compress (force=True) is unaffected — it never
enters this loop.

Tests: new TestOverflowWithCompactionDisabled (413 + 400 overflow don't
compress when disabled; control case still compresses when enabled).
Existing overflow-recovery tests updated to enable compaction explicitly
(they verify the recovery fires); fixture defaults flipped to True to
match production (compression.enabled defaults to True).

* fix(gemini): default native maxOutputTokens + strip OpenAI extra_body on Gemini endpoints

Two distinct failures hit users on the gemini provider with only Google
AI Studio keys set.

1. Truncation loop: build_gemini_request() only set maxOutputTokens when
   max_tokens was non-None. Hermes passes None to mean "unlimited", but
   Gemini's native generateContent does NOT treat an absent maxOutputTokens
   as full budget — it applies a low internal default and stops early with
   finishReason=MAX_TOKENS, truncating tool calls. The agent then retries
   3x and refuses the incomplete call. Now default to the published 65,535
   ceiling (shared by all current Gemini text models) when max_tokens=None.

2. HTTP 400 on Gemini endpoint: the chat_completions transport assembles
   profile extra_body (Nous portal 'tags', reasoning, provider prefs) and
   sends it via the OpenAI client to whatever base_url is resolved. When a
   profile that emits extra_body (e.g. Nous) is active but the endpoint is a
   native Gemini base_url — typical when only Google creds exist and a
   fallback/aux call lands on Gemini — Google rejects the unknown 'tags'
   field with a non-retryable 400. Strip all non-thinking_config extra_body
   keys when the resolved endpoint is native Gemini.

Verified E2E against real transport code: tags stripped on native Gemini,
preserved on Nous and the /openai compat endpoint; maxOutputTokens=65535
on None, explicit values respected.

agent/conversation_loop.py

@@ -2720,6 +2720,61 @@ def run_conversation(
                 # compress history and retry, not abort immediately.
                 status_code = getattr(api_error, "status_code", None)
 
+                # ── Respect disabled auto-compaction on overflow ──────
+                # Ported from anomalyco/opencode#30749.  When the user has
+                # turned auto-compaction off (``compression.enabled: false``),
+                # NO automatic compaction trigger may fire — including the
+                # provider/request-size overflow recovery paths below
+                # (long-context-tier 429, 413 payload-too-large, and
+                # context-overflow).  Without this guard the proactive
+                # threshold path correctly honours the setting (see the
+                # preflight check and the post-response ``should_compress``
+                # gate) but a provider overflow error would still silently
+                # compress + rotate the session, bypassing the user's
+                # explicit choice.  Surface a terminal error instead so the
+                # user can compact manually (``/compress``), start fresh
+                # (``/new``), switch to a larger-context model, or reduce
+                # attachments.  Forced compaction via ``/compress``
+                # (``force=True``) is unaffected — it never reaches this loop.
+                _overflow_reasons = {
+                    FailoverReason.long_context_tier,
+                    FailoverReason.payload_too_large,
+                    FailoverReason.context_overflow,
+                }
+                if (
+                    classified.reason in _overflow_reasons
+                    and not getattr(agent, "compression_enabled", True)
+                ):
+                    agent._flush_status_buffer()
+                    agent._vprint(
+                        f"{agent.log_prefix}❌ Context overflow, but auto-compaction is disabled "
+                        f"(compression.enabled: false).",
+                        force=True,
+                    )
+                    agent._vprint(
+                        f"{agent.log_prefix}   💡 Run /compress to compact manually, /new to start fresh, "
+                        f"switch to a larger-context model, or reduce attachments.",
+                        force=True,
+                    )
+                    logger.error(
+                        f"{agent.log_prefix}Context overflow ({classified.reason.value}) with "
+                        f"auto-compaction disabled — not compressing."
+                    )
+                    agent._persist_session(messages, conversation_history)
+                    return {
+                        "messages": messages,
+                        "completed": False,
+                        "api_calls": api_call_count,
+                        "error": (
+                            "Context overflow and auto-compaction is disabled "
+                            "(compression.enabled: false). Run /compress to compact manually, "
+                            "/new to start fresh, or switch to a larger-context model."
+                        ),
+                        "partial": True,
+                        "failed": True,
+                        "compaction_disabled": True,
+                    }
+
                 # ── Anthropic Sonnet long-context tier gate ───────────
                 # Anthropic returns HTTP 429 "Extra usage is required for
                 # long context requests" when a Claude Max (or similar)