fix: post-merge cleanup — eliminate internal/, fix tests (#819)

* fix: eliminate internal/, restore backfill tests, fix amp deadlock

- Delete internal/ entirely: migrate server.go to pkg/llmproxy/api/,
  remove duplicate cmd/ and tui/ files
- Restore backfillAntigravityModels method and tests from 7aa5aac
- Fix TestMultiSourceSecret_Concurrency goroutine leak (600s → 0.3s)
- Delete 2 empty test stubs superseded by pkg/llmproxy/ equivalents

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* WIP: save phase1-pkg-consolidation state

* fix: resolve remaining test failures across 5 packages

- Fix amp reverse proxy infinite loop (Rewrite bypassed Director URL routing)
- Add cursor models to static model definitions registry
- Fix extractAndRemoveBetas to skip non-string JSON array elements
- Fix trailing slash mismatch in OAuth base URL test
- Add response.function_call_arguments.done handler in Codex-to-Claude translator

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address PR review — return nil for empty codex output, enrich cursor model stub

- Return nil instead of empty slice when done event is deduplicated
- Populate standard fields on cursor model definition (Object, Type, DisplayName, Description)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: KooshaPari