Keep SRTX capture state retryable after start failures

[sylvesterkaczmarek]

Description

Keep SrtxCaptureState consistent when start_continuous_capture() fails.

start_or_extend() previously appended the new output path to its internal cache before restarting the SRTX capture. If the restart raised, the path remained cached even though capture never started. A later retry then saw the path as already present and returned immediately, turning a transient start failure into a permanently stuck output.

Build the candidate path list without mutating cached state and commit it only after start_continuous_capture() succeeds. If the first path for a sensor set fails, remove the empty cache entry so the next call follows the normal start path.

Validation

• adds a regression test with a first-call SRTX start failure
• verifies the failed output path is not cached
• retries the same output and verifies a second start attempt occurs
• verifies state is committed after the successful retry