fix(gateway): Slack approval UX in threads — block-size overflow + typed-prefix instruction text (#43444)

Two fixes for the reported Slack thread approval UX:

1. Slack Block Kit approval/confirm sends silently overflowed the
   3000-char section-block cap (flat 2900-char truncation + header +
   reason), so long execute_code approvals failed with invalid_blocks
   and fell back to the plain-text prompt with no buttons. Budget the
   command preview against the rendered fixed parts so blocks never
   exceed the cap (send_exec_approval + send_slash_confirm).

2. The text fallbacks told users to reply /approve — which Slack blocks
   inside threads and Matrix clients reserve client-side. Add a
   typed_command_prefix capability flag on BasePlatformAdapter
   (default "/"; Slack and Matrix set "!" to match their existing
   bang-prefix rewrite) and use it in the shared fallback prompt
   builders (exec approval, update prompt, destructive slash confirm,
   expensive-model confirm) plus Matrix's reaction-prompt text.
   The slash-confirm text-intercept now also accepts bang-prefixed
   replies (!always, !cancel) since those keywords aren't registered
   commands and the adapters' rewrite doesn't touch them.

gateway/platforms/slack.py

@@ -318,6 +318,11 @@ class SlackAdapter(BasePlatformAdapter):
 
     MAX_MESSAGE_LENGTH = 39000  # Slack API allows 40,000 chars; leave margin
     supports_code_blocks = True  # Slack mrkdwn renders fenced code blocks
+    # Slack blocks typed native slash commands inside threads ("/approve is
+    # not supported in threads. Sorry!").  The adapter rewrites a leading
+    # "!" to "/" for known commands (see _handle_slack_message), so "!" is
+    # the prefix that works everywhere — instruction text must show it.
+    typed_command_prefix = "!"
 
     def __init__(self, config: PlatformConfig):
         super().__init__(config, Platform.SLACK)
@@ -2692,19 +2697,26 @@ class SlackAdapter(BasePlatformAdapter):
             return SendResult(success=False, error="Not connected")
 
         try:
-            cmd_preview = command[:2900] + "..." if len(command) > 2900 else command
             thread_ts = self._resolve_thread_ts(None, metadata)
 
+            # Slack hard-caps a section block's text at 3000 chars; an
+            # oversized block fails the whole send with ``invalid_blocks``
+            # and the gateway falls back to the plain-text prompt (no
+            # buttons).  execute_code approvals embed the entire script in
+            # ``command``, so budget the preview against the fixed parts
+            # instead of a flat truncation that overflows once the header +
+            # reason are added.
+            header = ":warning: *Command Approval Required*\n"
+            reason = f"Reason: {description[:500]}"
+            budget = 3000 - len(header) - len(reason) - len("``````\n") - len("...")
+            cmd_preview = command[:budget] + "..." if len(command) > budget else command
+
             blocks = [
                 {
                     "type": "section",
                     "text": {
                         "type": "mrkdwn",
-                        "text": (
-                            f":warning: *Command Approval Required*\n"
-                            f"```{cmd_preview}```\n"
-                            f"Reason: {description}"
-                        ),
+                        "text": f"{header}```{cmd_preview}```\n{reason}",
                     },
                 },
                 {
@@ -2772,8 +2784,13 @@ class SlackAdapter(BasePlatformAdapter):
             return SendResult(success=False, error="Not connected")
 
         try:
-            body = message[:2900] + "..." if len(message) > 2900 else message
             thread_ts = self._resolve_thread_ts(None, metadata)
+            # Same 3000-char section-block cap as send_exec_approval: budget
+            # the body against the rendered title so the wrapper never pushes
+            # the block over the limit (overflow → invalid_blocks → no buttons).
+            _title = (title or "Confirm")[:150]
+            budget = 3000 - len(f"*{_title}*\n\n") - len("...")
+            body = message[:budget] + "..." if len(message) > budget else message
             # Encode session_key and confirm_id into the button value so the
             # callback handler can resolve without extra bookkeeping.
             value = f"{session_key}|{confirm_id}"
@@ -2783,7 +2800,7 @@ class SlackAdapter(BasePlatformAdapter):
                     "type": "section",
                     "text": {
                         "type": "mrkdwn",
-                        "text": f"*{title or 'Confirm'}*\n\n{body}",
+                        "text": f"*{_title}*\n\n{body}",
                     },
                 },
                 {