Lab misses controller Git fallback when runner cannot reach source host

[chubes4]

Problem

Homeboy Lab can terminate source materialization as unavailable when the selected runner cannot reach a private or proxied Git host, even though Homeboy owns a controller-routed Git bundle fallback for exactly this case.

materialize_git() records the remote Git stderr in an error hint while its top-level message is the generic changed-since materialization message. is_runner_git_auth_or_network_failure() examines only error.message, so it misses errors such as Could not read from remote repository and never activates controller bundle materialization.

Live evidence

On configured runner homeboy-lab:

• homeboy runner doctor homeboy-lab --scope lab-offload admits the runner.
• HTTPS access to the Enterprise host is unreachable.
• Git SSH access to the Enterprise host is unreachable.
• Homeboy reports source unavailability instead of routing materialization through the controller.

Persisted diagnostic runs:

• runner-exec-generic-homeboy-lab-1d67f535-be49-4444-9b5f-9d9290000036
• runner-exec-generic-homeboy-lab-dad8c46b-41ee-47fe-b7de-c6ab681a16ed

Expected behavior

When runner-side Git materialization fails because of authentication, proxy, DNS, or network reachability, Homeboy should classify the complete structured failure and transparently retry with controller-routed Git bundle materialization.

Acceptance criteria

• Failure classification includes the captured remote stderr/details, not only the generic top-level message.
• A regression test proves a generic materialization message plus network/auth stderr activates controller fallback classification.
• Existing non-network failures remain non-fallback failures.
• The behavior remains host- and organization-agnostic.

Related: #3586, #8131.