Audit SPP pushes with too many changed lines

What does this MR do and why?

Emits an audit event when Secret Push Protection skips scanning when a push exceeds the changed-lines limit that result in fail-open behavior.

• Adds the spp_too_many_lines audit event definition
• Logs the audit event from the too-many-lines path
• Updates audit event documentation and Secret Push Protection documentation
• Updates specs for the new audit logging behavior

References

Partially resolves #604787 (closed).

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.