fix(computer-use): address Copilot review on max_elements cap

Four findings from Copilot's review on PR #22891, all in the AX
elements-array cap added by 22fa1ed:

1. The truncation note ("response truncated to N of M elements") was
   appended unconditionally — including in the som/vision multimodal
   path, whose response carries a screenshot rather than an `elements`
   array. The note described a payload field that wasn't present.
   Moved the note into the AX-text branch where the array actually
   appears.

2. `_format_elements(cap.elements)` ran on the full untrimmed list with
   its own `max_lines=40` cap, so a caller passing `max_elements=10`
   would see summary lines referencing `#11..#40` even though the JSON
   `elements` array only held #1..#10. Format on `visible_elements`
   instead so the summary indices always exist in the response.

3. `_coerce_max_elements` enforced a lower bound but no upper bound,
   so `max_elements=10_000_000` silently disabled the safeguard and
   reintroduced the original context-blow-up. Added a hard cap
   (`_MAX_ALLOWED_MAX_ELEMENTS = 1000`) that clamps oversized values.

4. The schema string said "Default 100" but the property carried no
   `default` field, and claimed `max_elements` had no effect on som/
   vision while the image-missing fallback path can still return an
   elements array. Added `"default": 100`, `"maximum": 1000`, and
   clarified the fallback-path wording.

Each finding gets a regression test:

- test_capture_ax_clamps_oversized_max_elements_to_hard_cap
- test_capture_ax_summary_indices_match_returned_elements
- test_capture_multimodal_summary_omits_truncation_note
- test_schema_max_elements_documents_default_and_upper_bound

Verified with `pytest tests/tools/test_computer_use.py` (53 passed,
including the 5 new cases). Confirmed each new test fails on the
pre-fix code path before applying the production change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/computer_use/schema.py

@@ -79,18 +79,23 @@ COMPUTER_USE_SCHEMA: Dict[str, Any] = {
                 "type": "integer",
                 "description": (
                     "Optional cap on the AX `elements` array returned by "
-                    "`action='capture'`. Default 100. Dense UIs (Electron "
-                    "apps such as Obsidian or VS Code, JetBrains IDEs) can "
-                    "publish 500+ AX nodes — capping prevents a single "
-                    "capture from blowing session context. When the cap "
-                    "trims the response, `total_elements` and "
-                    "`truncated_elements` are surfaced in the result so "
-                    "you can re-call with `app=` to narrow scope or raise "
-                    "`max_elements` when the full tree is required. Has no "
-                    "effect on `mode='som'` / `mode='vision'` (those return "
-                    "a screenshot, not the elements array)."
+                    "`action='capture'`. Default 100, hard maximum 1000. "
+                    "Dense UIs (Electron apps such as Obsidian or VS Code, "
+                    "JetBrains IDEs) can publish 500+ AX nodes — capping "
+                    "prevents a single capture from blowing session "
+                    "context. When the cap trims the response, "
+                    "`total_elements` and `truncated_elements` are "
+                    "surfaced in the result so you can re-call with "
+                    "`app=` to narrow scope or raise `max_elements` when "
+                    "the full tree is required. Has no effect on "
+                    "`mode='som'` / `mode='vision'` when a screenshot is "
+                    "included in the response; only the rare image-"
+                    "missing fallback returns an `elements` array and is "
+                    "subject to the cap."
                 ),
+                "default": 100,
                 "minimum": 1,
+                "maximum": 1000,
             },
             # ── click / drag / scroll targeting ────────────────────
             "element": {