MCP Track A Phase 2: add capability probe, richer metadata, and result policies

[Bionic711]

Summary

Implement Track A Phase 2 from #1013: make outbound MCP discovery, capability metadata, schema handling, and result handling more robust.

Scope

• Add an MCP compatibility probe endpoint or discovery mode.
• Capture transport, auth method, tool count, capability hints, and warnings.
• Preserve richer cached tool metadata including original name, normalized function name, description, input schema, annotations where available, output schema where available, and structured-content hints where available.
• Keep compatibility with existing mcp_tools entries.
• Add opt-in or warning-only input schema validation before invocation.
• Replace the single hard-coded truncation behavior with explicit result policies.
• Add UI warnings for empty tool lists, failed discovery with cached tools, duplicate normalized function names, unsupported prompts/resources, and overly broad schemas.

Acceptance Criteria

• Probe/discovery returns useful diagnostics without leaking secrets.
• Existing cached mcp_tools remain readable and valid.
• Richer tool metadata is normalized and validated.
• Schema validation is disabled or warning-only by default to avoid breaking dynamic tools.
• Result-size policy defaults to truncate and supports future policy expansion.
• Functional/UI tests cover probe, metadata, warnings, schema validation mode, and result policy behavior.

Notes

Parent: #1013
Planning doc: docs/explanation/features/MCP_PLUGIN_ROBUSTNESS_PLAN.md
Priority: P1
Size: M