Hash only the owner execute bit on source files

[tinder-maxwellelliott]

Summary

• Narrow SourceFileHasher to check only PosixFilePermission.OWNER_EXECUTE instead of OR'ing owner/group/others. Pulled into a private isOwnerExecutable helper to avoid duplicating the try/catch at both call sites.
• Add regression test testHashIgnoresGroupAndOthersExecuteBits that hashes the same file with mode rw-------, rw---x---, and rw------x and asserts all three hashes are identical.
• Existing testHashExecutableBitChange (issue Missing bazel-diff support for change executable bit of files #325) still passes since it only toggles OWNER_EXECUTE.

Why

Cross-referencing how a sibling tool handles this: bazel-contrib/target-determinator shipped commit 5bfae36 ("fix(hash_cache): only take user execute bits into account") for the same reason:

There are some cases where files are initially created with some permissions (e.g. 775) but are then created by git with different permissions (e.g. 755) when cloning, checking out a branch or creating a worktree. Such differences don't matter to git nor to Bazel; consequently, they should not change the hash of a source file in target-determinator. The execution bit for the user (e.g. 744 vs 644) does matter, though, and should be part of the hash.

Git's index only records 100644 vs 100755 — the owner execute bit. Group/others execute bits aren't represented and can flip between checkouts under different umasks. Folding them into the hash adds noise without buying any signal that the build cares about.

The current OR-of-three logic was introduced in #331 (fix for #325). #325's example use case (toggling the owner execute bit on a script under a filegroup) is preserved by this change.

Test plan

• bazel test //cli:SourceFileHasherTest passes locally (testHashExecutableBitChange and the new testHashIgnoresGroupAndOthersExecuteBits both green).
• bazel test //cli:BuildGraphHasherTest //cli:TargetHashTest passes locally — no broader regressions.
• CI on ubuntu+macos × Bazel 7/8/9.

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