fix: make consolidated method errors actionable

[Tethys0]

Fixes #2712

Summary

Consolidated method-dispatch tools now return actionable errors that list the supported methods when a caller supplies an unknown method. pull_request_review_write also reports the missing required method parameter before decoding the remaining request fields.

User impact

MCP clients can recover from a typo or omitted dispatch method without guessing the tool's internal routing options. The behavior is applied consistently to issue, pull request, Actions, Projects, sub-issue, and UI dispatchers.

What changed

• Added a shared supported-method error formatter.
• Replaced bare unknown-method errors in the affected dispatchers.
• Added the missing method validation in pull_request_review_write.
• Added the sub_issue_write method enum so its schema matches runtime dispatch.
• Added focused handler tests covering all affected dispatchers and the missing-method case.

Validation

• go test ./... - 3398 passed
• go vet ./... - clean
• ./script/lint - 0 issues
• git diff --check - clean

AI assistance was used for code exploration and test scaffolding; I reviewed and own the implementation and can explain the behavior and trade-offs.