2291: backport: squash and use `cherry-pick`, not rebase r=Urhengulas a=jyn514

This hits many fewer conflicts when backporting. Now, instead of requiring every commit to backport cleanly, we only require the diff between `main` and the PR to backport cleanly. This decreases the fraction of PRs that need to be submitted manually, and also reduces the manual work needed to backport PRs when a manual backport is needed.

There are also various other changes to make the backporting scripts more maintainable and testable; see each individual commit for details. Unlike before, this does not mess with the current git state; running `git cherry-pick --abort` puts you back in exactly the same state you were in before you ran `one.py`.

See #2301 for an example PR opened by this automation, and https://github.com/ferrocene/ferrocene/actions/runs/24351997960/job/71108663815 for a sample job output.

---

## Testing

Here is a sample session using `one.py` to do a manual backport for the upcoming release:
```console
$ git checkout origin/release/1.95
HEAD is now at a31dd5c Merge #2258
$ git checkout jyn/backport-automation ferrocene/tools/backport ferrocene/tools/automations-common
$ ferrocene/tools/backport/one.py 2245
ERROR: this script cannot be run when you have staged changes; try running `git reset`
$ git reset
Unstaged changes after reset:
M	ferrocene/tools/automations-common/src/automations_common/automated_prs.py
M	ferrocene/tools/backport/all.py
M	ferrocene/tools/backport/one.py
$ ferrocene/tools/backport/one.py 2245                                                                                                                                                                                                         17:45
warning: if no API token is set in the GITHUB_TOKEN env var, requests may be rate-limited
INFO: backporting #2245 to a31dd5c (please do not CTRL-C)

Auto-merging .circleci/workflows.yml
Auto-merging ferrocene/ci/scripts/setup-uv.sh
CONFLICT (content): Merge conflict in ferrocene/ci/scripts/setup-uv.sh
error: could not apply d3faae633305... Backport #2245

ERROR: Encountered a merge conflict backporting #2245

INFO: attempting to auto-solve conflicts with mergiraf
WARN Mergiraf: Could not find a supported language for ferrocene/ci/scripts/setup-uv.sh
HELP: Fix the conflicts, run 'git add/rm <filepath>', then run 'git cherry-pick --continue'.
NOTE: Running 'git cherry-pick --abort' will put you back in your original git state.

NOTE: conflicts exist in the following locations:
ferrocene/ci/scripts/setup-uv.sh:7-17
NOTE: 'HEAD' in the conflicted files refers to your current branch, a31dd5c

$ git diff --name-status --cached
M       .circleci/workflows.yml
U       ferrocene/ci/scripts/setup-uv.sh
$ git status | head -n5
HEAD detached at origin/release/1.95
You are currently cherry-picking commit d3faae633305.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)
```

(After this lands on beta, the `git checkout ferrocene/tools && git reset` lines will no longer be needed.)

2336: Fix several bugs in `ferrocene::prevalidated` r=Urhengulas a=jyn514

Thanks to `@Urhengulas` for catching these.

1. The THIR pass didn't check associated items at all. This is fixed in the first commit and tested in `tests/ui/ferrocene/lint-prevalidated/thir-impl-item.rs`. The second commit adds the missing items to libcore.
2. The post-mono pass didn't check dynamic casts. I had correctly "failed closed" here -- the THIR pass would ICE if it wasn't able to check a cast — but because of 1. there were several casts in libcore that weren't caught, including in libcore. The ICE is fixed in the third commit and the post-mono pass is extended in the fourth. This is tested in `tests/ui/ferrocene/lint-prevalidated/dyn-cast-post-mono.rs`.
3. The post-mono pass can't handle MIR inlining. I tried to get this working (see https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/SourceScopeData.20-.3E.20SourceScope/with/590200295), but wasn't able to figure out how to do it reliably. Instead I just disable mir inlining in the 5th commit. This is tested in `tests/ui/ferrocene/lint-prevalidated/mir-inlining.rs`.

Co-authored-by: Jynn Nelson <[email protected]>
Co-authored-by: Urhengulas <[email protected]>