[Test] Add comprehensive functional tests for PR 6014 (url_redirector chain-aware...

[Test] Add comprehensive functional tests for PR 6014 (url_redirector chain-aware cache)

Add complete test coverage for url_redirector PR 6014 features:

Test Suites (33 tests total):
- 162_url_redirector: Enhanced with chain resolution tests (4 tests)
- 163_url_redirector_chain: Core PR 6014 features (7 tests)
- 164_url_redirector_pr6014: Detailed PR 6014 functionality (8 tests)
- 165_url_redirector_cache: In-depth cache behavior (8 tests)
- 166_url_redirector_config: Configuration variations (6 tests)

Features Tested:
- Chain-aware cache with per-hop Redis entries
- ^hop: and ^nested: marker behavior
- Intermediate hop injection for downstream modules
- Self-healing cache (^nested: → ^hop: upgrade)
- Separate timeout configuration (timeout, http_timeout, redis_timeout)
- save_intermediate_redirs setting (redirectors/non_redirectors)
- Full host path in symbols (host1->host2->...->hostN)
- Cache cycle detection
- Multiple redirect chains in single message

Test Infrastructure:
- 2 new test messages (chain_redirect.eml, chain_multipart.eml)
- 2 new config files (url_redirector_chain.conf, url_redirector_no_intermediate.conf)
- Enhanced dummy_http.py with 3-hop chain endpoints (/chain1, /chain2, /chain3)
- Complete test documentation (PR6014_TESTS.md)
- Test results summary (PR6014_TEST_RESULTS.md)

All 33 tests pass successfully with build, C/C++, and Lua unit tests.