feat(agent): budget model-visible MCP schemas

[dajiaohuang]

Summary

• add an opt-in byte budget for model-visible MCP tool schemas, disabled by default
• preserve every built-in tool and leave the registered/executable MCP tool set unchanged
• choose a deterministic MCP subset from the latest visible user request and keep it stable for the run
• fail open to all MCP schemas when intent is ambiguous, schemas are malformed, or the strongest match cannot fit

Draft scope

This draft explores the conservative budgeted-view option from the issue without adding another model, embedding, or discovery call. The ASCII lexical matcher intentionally fails open for non-English or ambiguous turns; disabling the setting restores existing behavior.

Relates to #5298.

Validation

• focused runner, configuration, and selector tests: 129 passed
• Ruff: passed
• BasedPyright: 0 errors, 0 warnings, 0 notes
• full suite before the final exact-byte adjustment: 6372 passed, 48 skipped, with 5 unrelated Windows/date-boundary baseline failures; the focused suite, Ruff, and BasedPyright were rerun after that adjustment
• git diff --check: passed