internal/analysis/driverutil: report partial success in ApplyFixes

[AshvinBambhaniya]

When the fix tool applies some suggested fixes but skips others due to
conflicts, it previously returned a non-zero exit code.

This caused the 'go fix' command to assume the tool failed, leading it
to discard the partial progress (the FixArchive) and not update the
source files. On subsequent runs, the tool would see the original code
again and repeat the same partial fixes, creating an infinite loop.

This change modifies ApplyFixes to return nil (success) if at least
one fix was applied and no I/O errors occurred. This ensures that
partial progress is saved to disk, breaking the non-convergence loop.

Fixes #78493

[gopherbot]

This PR (HEAD: 470d26a) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/787260.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.