Improve "unknown method" errors to list supported methods

[gustavo-sec]

Closes #2712.

The consolidated method-dispatch tools returned a bare unknown method: <x> with no hint of valid values, and each tool's valid method set lived only in its schema enum (or, for sub_issue_write, only in prose). labels.go already listed the supported methods on an unknown value; this brings the rest in line and makes the advertised methods the single source of truth.

What changed

• Each tool's method set is a single []string that feeds both the input-schema enum (methodEnum) and the unknown-method error (unknownMethodError), so the advertised methods and the accepted methods cannot drift apart.
• An unknown method now lists the supported set: unknown method: x. Supported methods are: ....
• pull_request_review_write decoded args with mapstructure.WeakDecode (it does not use RequiredParam like the other tools), so an omitted method surfaced as unknown method: (empty value). It now reports missing required parameter: method, consistent with every other tool.
• sub_issue_write advertised its method options only in the description; it now declares the enum like every other method-dispatch tool.

Affected tools: pull_request_read, pull_request_review_write, issue_read, sub_issue_write, actions (list/get/run), projects (list/get/write), ui_get.

Testing

• go test ./... — green.
• golangci-lint run (v2.9.0) — 0 issues.
• script/generate-docs — no changes.
• Tool snapshots regenerated: only sub_issue_write changed (it gained the advertised method enum).
• Added unit tests for the helpers and for the review-write empty/unknown-method paths; tightened the existing projects/ui_get unknown-method assertions.
• Verified live over stdio against a running server: omitted method → missing required parameter: method; method: "bogus" → unknown method: bogus. Supported methods are: ....