fix(computer-use): cap AX elements array to prevent context blowup (#22865)

`computer_use(action='capture', mode='ax')` returned the full AX element
list verbatim in the JSON response. Dense Electron / Obsidian / JetBrains
UIs publish 500+ AX nodes (one reproduction in #22865 returned 597
elements against Obsidian), so a single capture could consume enough
context to trigger compression failures or render the session unusable.
The human-readable `_format_elements` summary is already capped at 40
lines, so the truncation gap was invisible to anyone reading the summary
output.

Add a `max_elements` argument to the tool schema, default 100, that
trims the AX `elements` array. When the cap fires, the response surfaces
`total_elements` and `truncated_elements` and appends a "raise
max_elements or pass app= to narrow" hint to the summary so the model
knows the JSON view is partial and can re-issue with a tighter scope.

Validation is centralized in `_coerce_max_elements`: missing /
non-integer / sub-1 inputs fall back to the default cap, so the
protection can never be silently disabled by a malformed tool-call
argument. The cap only affects AX-mode JSON; `mode='som'` and
`mode='vision'` keep returning a screenshot + image-aware summary
unchanged.

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

tools/computer_use/schema.py

@@ -75,6 +75,23 @@ COMPUTER_USE_SCHEMA: Dict[str, Any] = {
                     "frontmost app's window or the whole screen."
                 ),
             },
+            "max_elements": {
+                "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)."
+                ),
+                "minimum": 1,
+            },
             # ── click / drag / scroll targeting ────────────────────
             "element": {
                 "type": "integer",