fix: invalidate tool schema cache on ToolListChangedNotification; paginate all pages in _validate_tool_result

[atishay2]

Summary

Two correctness bugs in ClientSession that cause tool output schema validation to silently use stale or missing schemas.

Bug 1: ToolListChangedNotification does not clear the tool schema cache

_received_notification had no handler for ToolListChangedNotification, so _tool_output_schemas was never invalidated when the server signalled a tool list change. The next call_tool invocation would validate structured_content against the old (stale) schema:

• False negative: a tool whose schema changed from {integer} to {string} would have valid string output rejected against the cached integer schema.
• Silent acceptance: a tool whose schema became stricter would still pass validation because the old looser schema was still in the cache.

Fix: handle ToolListChangedNotification in _received_notification by clearing _tool_output_schemas. The cache is repopulated lazily on the next call_tool.

Bug 2: _validate_tool_result only fetches the first page of list_tools()

When a tool name was not in _tool_output_schemas, a single await self.list_tools() call was made (no arguments → first page only). If the tool lived on page 2 or later, it was never found in the cache, the schema lookup fell through to the warning path, and validation was silently skipped — invalid structured_content was returned to the caller without any error.

Fix: add a pagination loop in _validate_tool_result that continues fetching pages until the tool is found or all pages are exhausted.

Changed files

• src/mcp/client/session.py — two targeted fixes (4 new lines each)
• tests/client/test_output_schema_validation.py — two new test cases that fail without the fix and pass with it

Test plan

• uv run --frozen pytest tests/client/test_output_schema_validation.py -v — all 7 tests pass
• uv run --frozen pytest tests/client/ -v — 190 passed, 1 xfailed, no regressions
• uv run --frozen pyright src/mcp/client/session.py — 0 errors
• uv run --frozen ruff check src/mcp/client/session.py tests/client/test_output_schema_validation.py — all checks passed

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