fix(tools): list supported methods in unknown-method dispatch errors

[syf2211]

Summary

Improve consolidated method-dispatch MCP tools to return actionable errors when an unrecognized method is provided, matching the existing label_write behavior.

Motivation

Fixes #2712. Tools such as pull_request_read, pull_request_review_write, issue_read, sub_issue_write, actions_*, projects_*, and ui_get previously returned bare unknown method: <value> errors. When method was omitted from pull_request_review_write, callers saw unknown method: with an empty value instead of a required-parameter error.

Changes

• Add shared methodEnum() and unknownMethodError() helpers in pkg/github/params.go
• Declare per-tool method slices as the single source for schema enums and runtime validation
• Update consolidated tools to use the helpers in their default branches
• Validate required method in pull_request_review_write before WeakDecode routing
• Add regression tests for helpers and pull_request_review_write error paths
• Update sub_issue_write toolsnap enum

Tests

go test ./pkg/github/... -count=1

Result: PASS (0.161s)

Notes

• Rebased on current main; issue_read retains get_parent support.
• label_write already had inline supported-method messaging and was left unchanged to keep scope minimal.
• Related open PR Improve "unknown method" errors to list supported methods #2713 appears conflicted/stale; this branch is a fresh implementation on current main.