refactor(pull-requests): migrate consolidated PR tools to typed inputs and outputs

[SamMorrowDrums]

Summary

Moves the consolidated pull request tools in pkg/github/pullrequests.go to NewTool[In,Out] with concrete inputs and output DTOs. Stacked on #3395.

Why

Continues the typed-output stack. Modern-protocol clients get schema-valid structuredContent, while legacy and unknown protocols get the same output as before.
Fixes #

What changed

• Typed tools: pull_request_read, create_pull_request, update_pull_request, merge_pull_request, update_pull_request_branch, pull_request_review_write (both the default and the thread-resolution-reason variants), add_comment_to_pending_review and add_reply_to_pull_request_comment.
• New consolidated_pull_request_types.go with:
  • Concrete inputs.
  • An explicit oneOf union for the pull_request_read methods, with a diff variant and a null variant.
  • Write, merge, branch and reply output DTOs, including the awaiting-form result and the reply/reaction union.
  • Legacy argument normalizers.
• The legacy handlers never ran schema validation, so the normalizers repeat each handler's checks in the same order with the same error text. They also keep the legacy coercions:
  • numeric strings and whole floats;
  • review_write keys matched case-insensitively, plus its WeakDecode behaviour;
  • the commentId BigInt and < 1 errors;
  • "unknown method" errors;
  • the create form deferral when title/head/base are missing.
• Legacy text output is byte-for-byte unchanged. structuredContent is sent only on 2026-07-28 and omitted on legacy and unknown protocols.
• Tests in typed_consolidated_pull_request_outputs_test.go cover:
  • call and list on modern, legacy and unknown protocols;
  • the feature variant;
  • the awaiting form;
  • legacy error and coercion cases, checked against a probe of the parent c918f4b;
  • API errors;
  • schema resolution and conformance of zero values.
• Updated toolsnaps, plus new _typed snapshots.

MCP impact

• No tool or API changes
• Tool schema or behavior changed — the 8 tools now declare an output schema. Input schemas and legacy text are unchanged.
• New tool added

Prompts tested (tool changes only)

• Covered by in-process MCP client tests rather than live prompts. Examples: "Get the diff/files/reviews of PR Port CLI Server #1", "Merge PR Port CLI Server #1 with squash", "Reply to review comment 42 with a heart reaction".

Security / limits

• No security or limits impact — the same API calls, permissions and content filtering as before.
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a 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

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint — passed (0 issues)
• Tested locally with ./script/test — passed

Also run:

• UPDATE_TOOLSNAPS=true go test ./... — passed
• script/generate-docs — no doc changes
• git diff --check — clean

Known CI: an inherited macOS flake from #3395 in typed_granular_issue_outputs_test.go:389 (the int64 overflow message differs on macOS).

Docs

• Not needed — script/generate-docs produced no changes.
• Updated (README / docs / examples)