Reply cleanly on unhandled credential errors in run channel

[midigofrank]

Description

RunChannel.handle_credential_error/5 pattern-matched a fixed set of error tags with no fallback, so an unexpected error term — most notably an OAuth provider timeout during token refresh, which surfaces as an untyped transport error — raised FunctionClauseError and crashed the channel mid-fetch:credential, dropping the worker's connection.

Add a fallback clause that logs the unexpected term and replies with a clean error so the worker receives a response instead of losing the connection.

Closes #4853

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[github-actions]

Security Review ✅

• S0 (project scoping): N/A — the diff only adds a fallback error-reply clause in run_channel.ex:553-562; no new queries or entrypoints for project-scoped data.
• S1 (authorization): N/A — no new actions added; the existing fetch:credential authorization path is unchanged, the new clause only formats an error reply.
• S2 (audit trail): N/A — the fallback only logs and replies with an error message; no insert/update/delete on config resources.

[midigofrank]

@elias-ba I'm merging this in. I know you were interested in some metrics? I haven't added any, this change simply stops the run channel from crashing when it receives an unknown error from the Credentials.resolve_credential_body/2. There are some audits in place for tracking these errors: https://github.com/OpenFn/lightning/blob/main/lib/lightning/credentials/audit.ex#L80