refactor(open-sse): registry consolidation + DRY media/oauth/adhoc cleanup

- Single-source registry: oauth clientId/tokenUrl, usage URLs, image/embed
  configs, search defaultModel, codex fixedPort, google token url derive.
- Remove 29 unused OmniRoute providers (registry 100→71); media intact.
- De-adhoc: codex literals → registry format/oauth flags; reasoningInject,
  image/embed openrouter headers + xai bodyFields config-driven.
- Add REGISTRY_TEMPLATE.js + expand PROVIDER_DEFAULTS/schema JSDoc.
- Baselines updated; PROVIDERS 62 + alias 90 byte-for-byte, golden snapshots.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/__baseline__/current.json

@@ -1 +1 @@
-{"numTotalTestSuites":222,"numPassedTestSuites":205,"numFailedTestSuites":17,"numPendingTestSuites":0,"numTotalTests":827,"numPassedTests":781,"numFailedTests":26,"numPendingTests":20,"numTodoTests":0,"snapshot":{"added":0,"failure":false,"filesAdded":0,"filesRemoved":0,"filesRemovedList":[],"filesUnmatched":0,"filesUpdated":0,"matched":153,"total":153,"unchecked":0,"uncheckedKeysByFile":[],"unmatched":0,"updated":0,"didUpdate":false},"startTime":1781358085146,"success":false,"testResults":[{"aLine truncated
+{"numTotalTestSuites":206,"numPassedTestSuites":189,"numFailedTestSuites":17,"numPendingTestSuites":0,"numTotalTests":572,"numPassedTests":527,"numFailedTests":26,"numPendingTests":19,"numTodoTests":0,"snapshot":{"added":0,"failure":false,"filesAdded":0,"filesRemoved":0,"filesRemovedList":[],"filesUnmatched":0,"filesUpdated":0,"matched":0,"total":0,"unchecked":0,"uncheckedKeysByFile":[],"unmatched":0,"updated":0,"didUpdate":false},"startTime":1781412569603,"success":false,"testResults":[{"asserLine truncated