Restore MCPMethodInfo argument compatibility with lazy caching

[Copilot]

Summary

Restore MCPMethodInfo argument compatibility fields and cache decoded arguments on demand. Preserve independent extraction of owner and repo when one value has an invalid type.

Why

Consumers need the established Owner, Repo, and Arguments fields without forcing every request to materialize its full argument payload.

What changed

• Add Owner, Repo, and lazily populated Arguments fields.
• Cache decoded arguments and errors; serialize concurrent DecodeArguments() calls.
• Document that decoded arguments are shared, read-only, and argument fields must not change after decoding.
• Cover invalid field types, duplicate keys, cache behavior, and concurrent calls.

MCP impact

• No tool or API changes
  MCP tool schemas and wire behavior are unchanged; this restores Go-side MCPMethodInfo compatibility.
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• Not applicable; no MCP tools changed.

Security / limits

• Auth / permissions considered
  Scope challenge behavior and invalid-argument fallback remain unchanged.
• Data exposure, filtering, or token/size limits considered
  Decoded arguments are shared and documented as read-only; request size limits are unchanged.
• No security or limits impact

Tool renaming

• I am renaming tools as part of this PR (e.g. as part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint — unable to complete because golangci-lint could not be downloaded.
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Daigrin]

Approve