[flagd] Fix in-process provider to use FLAGD_SYNC_PORT (not FLAGD_PORT) for port configuration (backwards compatibility)

[aepfli]

Background

Recent updates to flagd and related tests in flagd-testbed clarified the use of environment variables for ports:

• FLAGD_PORT is for remote/server mode.
• FLAGD_SYNC_PORT should be used for in-process/sync providers.

Currently, the in-process provider uses the wrong environment variable (FLAGD_PORT).

Required Changes

• Fix provider configuration logic to use FLAGD_SYNC_PORT for in-process provider setups, and fall back to FLAGD_PORT for backwards compatibility.
• Update any relevant documentation and configuration references.
• Ensure test coverage, ideally using scenarios matching those in flagd-testbed (with @legacy-sync-port tag for backwards compatibility coverage).

References

• Parent: [BUG] Conceptional issue with Environment names and configuration options, for flagd and the providers flagd#1573
• Testbed: Flagd in_process env var should be FLAGD_SYNC_PORT instead of FLAGD_PORT flagd-testbed#266