Fix driver test probes for local URLs and persisted secrets

[segran2]

Summary

• allow Test connection for configured loopback driver URLs only when the requested probe matches the saved enabled driver and URL
• wire driver test probes to the original driver's persisted secret state so OAuth drivers reuse refreshed tokens
• keep probe secret writes under the original driver name instead of the temporary _test* name
• add regression coverage for configured loopback access and OAuth secret reuse/persistence

Fixes #1502.

Verification

• go test ./internal/api -run 'TestHandleDriverTest|TestConfiguredProbeLoopbackHost|TestRejectUnsafeProbeTargets'
• built linux/arm64 package v0.139.0-beta.10000 and installed on FTW at 192.168.1.88
• verified myUplink Test connection returns ok:true with telemetry and no invalid_grant in journal

[segran2]

Hardware validation completed on ARM64 FTW with v0.139.2-beta.4 built from PR head 7dba6aa.

Validated MyUplink persisted-secret reuse without running "Connect to MyUplink" again:

• FTW started v0.139.2-beta.4 as a native trial and committed it successfully.
• Existing MyUplink driver auto-detected the device and reached ready (read-only).
• Ran Test connection directly without a new OAuth login.
• First probe created __test_myuplink_dlvmy877u0hd, auto-detected the device, reached ready (read-only), and was removed normally.
• Reloaded the web UI and ran Test connection again, still without Connect/OAuth.
• Second probe created __test_myuplink_dlvmyx14zxyt, again reached ready (read-only), and was removed normally.
• No invalid_grant was logged.

This verifies the original issue on real hardware: Test connection can reuse the installed driver's persisted MyUplink authentication across UI reloads instead of requiring Connect/login first.

Additional verification on this head:

• go test ./internal/api — pass
• native ARM64 runtime healthy after activation

[segran2]

Ready to merge — hardware validated and all checks passing.

Final status:

• Current PR head 8f47905a is cleanly mergeable with the current base branch.
• All applicable GitHub checks pass (10 successful, 4 skipped).
• No merge conflicts.
• MyUplink Test connection was hardware-validated on ARM64 FTW without running Connect/OAuth again, including after a web UI reload.
• Two independent __test_myuplink_* probes reached ready (read-only) and were removed normally.
• No invalid_grant observed.

The superseded UI workaround #1514 has been closed. This PR addresses the root cause via persisted driver secret reuse.

Maintainer: please merge when ready.