fix(errors): use rate_limit for HTTP 429

[hussainemiola942-cloud]

Overview

Updates API error normalization so HTTP 429 responses use the issue-specified rate_limit kind instead of rate_limited.

Related Issue

Closes #381

Changes

• Renamed the ApiErrorKind value for HTTP 429 from rate_limited to rate_limit.
• Updated normalizeApiError to return rate_limit for 429 responses.
• Updated the existing error normalization test expectation for 429 responses.

Verification Results

Command	Result
npm.cmd test -- --run tests/errors.test.ts	Passed: 11 tests
npm.cmd test	Failed due to unrelated existing failures: 37 files passed, 10 files failed; 216 tests passed, 18 failed
npm.cmd run typecheck	Failed due to unrelated existing syntax errors in src/lib/session-sse.ts, tests/export-filename.test.ts, and tests/redirect.test.ts
npm.cmd run lint	Failed before project linting because @typescript-eslint/no-unsafe-assignment requires typed parser options in eslint.config.mjs
git diff --check	Passed

Acceptance Criteria

Criteria	Status
409 responses normalize to a conflict kind	Existing behavior confirmed
429 responses normalize to the issue-specified rate limit kind	Done
Tests cover the 429 kind contract	Done

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