Add --dry-run flag to pyrefly infer

[nitishagar]

Summary

pyrefly infer rewrites files in place and always exits 0, so it cannot be used as a CI check for whether the formatter would make any changes.

This adds a --dry-run flag to pyrefly infer that runs the full inference, prints what changes would be made, writes nothing, and exits 0 if no changes would be made or 1 if any changes would be made — enabling CI check mode.

The change set is already computed by the existing read-only phase of run_inner; dry-run simply skips the two file writes (gated on a new dry_run flag on InferFlags), logs the planned changes per file, and returns UserError when any changes are pending. Non-dry-run behavior is unchanged.

$ pyrefly infer --dry-run file.py
 INFO file.py: would add 1 annotation(s) and 0 import(s)
$ echo $?
1
# file.py is unchanged

Fixes #3920.

Test plan

• cargo build -p pyrefly
• cargo test -p pyrefly dry_run — 12 passed, asserting both the exit status (UserError when changes, Success when none) and that the file is left byte-identical.
• cargo clippy -p pyrefly --tests
• Manual: pyrefly infer --dry-run on an unannotated file leaves it unchanged and exits 1 with the planned change printed; without --dry-run the file is annotated and exits 0.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅