fix: preserve transport base path when building REST polling URL

[dev-satoshi]

Description

Fixes REST polling URL construction so janus_client no longer assumes a hardcoded /janus path.
The plugin now preserves the provided transport base endpoint path and appends only the session segment, enabling API Gateway / reverse proxy deployments with custom public paths.

Changes

• Updated REST polling URL construction in JanusPlugin to preserve transport.url path
• Added JanusPlugin.buildPollingUri(...) helper for consistent URI composition
• Added tests for:
  • custom base path support
  • trailing slash normalization
  • query parameter merging
• Removed unused extractDomainFromUrl from utils.dart
• Updated CHANGELOG.md (2.4.3)

Benefits

• Custom base paths are supported (e.g. /api/video/janus, /custom/path)
• Avoids URL mismatches in API Gateway / reverse proxy topologies
• Reduces client-side workarounds for environment-specific routing

Testing

• Ran test/janus_plugin_test.dart
• Added/validated buildPollingUri coverage for path/query edge cases
• Confirmed no lint errors in modified files
• Confirmed session creation and REST polling work without requiring a hardcoded /janus root

Breaking Changes

None — backward compatible.

Notes

API docs were not regenerated in this PR to keep review focused; a docs-only follow-up PR can be provided if needed.

Closes #198