fix: apply server_replay_extra policy even after replay map is exhausted - #8365

Description

When server replay is configured without server_replay_reuse (the default), consuming the final replayable response empties the replay map. Every later request then bypasses server_replay_extra because the extra-request policy is gated behind if self.flowmap:.

This contradicts the option's documented contract: server_replay_extra controls requests during replay for which no replayable response was found. It also makes numeric policies such as 404 stop applying silently after the last saved response is consumed.

Changes

• mitmproxy/addons/serverplayback.py: Move the server_replay_extra handling (kill, return synthetic status code) outside the if self.flowmap: guard. After a matched replay flow is served, the function returns early. If no match is found (or the flowmap is empty), the configured extra-request policy is applied.

Tests

• test/mitmproxy/addons/test_serverplayback.py: Add test_server_playback_extra_after_exhaustion — loads one flow, consumes it, then verifies that the configured server_replay_extra policy (204, 400, 404, 500) is still applied to the next request.

Closes #8363