fix(autofix): Gracefully handle ResourceNotFound for missing branches/repos

[sentry]

This PR addresses noisy Sentry errors (ResourceNotFound) occurring in autofix agent functions when attempting to fetch information for GitHub repositories or branches that no longer exist.

Previously, _build_base_shas_metadata and _resolve_default_branch in src/sentry/seer/autofix/autofix_agent.py used a generic except Exception: block. This caught scm.errors.ResourceNotFound (which is raised when GitHub returns a 404 for a missing resource) and logged it as an error, creating unnecessary Sentry issues.

This fix introduces a specific except ResourceNotFound: handler before the generic exception. When a ResourceNotFound error occurs, it is now caught and logged at an info level instead of an exception level. This allows autofix to gracefully handle cases where a linked repository or branch has been deleted or renamed, reducing Sentry noise while still providing visibility into these events.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

Fixes SENTRY-5RA5

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀