Pull the max-prefix-len computation into its own helper and rename the matched-token argument

[fzyzcjy]

In Req.init_next_round_input, the three-line max_prefix_len computation is
moved into a small helper _compute_max_prefix_len(input_len) so the
remaining body reads fill_ids[:self._compute_max_prefix_len(input_len)]
in one line. The resulting slice is then bound to a more specific name
token_ids_to_match (it is what gets passed as the prefix-match key on
the radix tree) instead of the generic token_ids.

---

CI States

Latest PR Test (Base): ⏳ Run #26070393056
Latest PR Test (Extra): ⚠️ Not enabled -- add run-ci-extra label to opt in.

[fzyzcjy]

/tag-and-rerun-ci

[gemini-code-assist]

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[fzyzcjy]

❌ verify-rebased: FAIL

PR head tree is NOT byte-equal to rebase(tom_refactor_202605a/primary/nonmech_scheduler onto upstream/main). Do not merge until you understand the diff.

	SHA	Tree
PR head (tom/pr_chain/tom_refactor_202605a/primary/nonmech_scheduler/compute-max-prefix-len)	1e5ab7e2667c	975b53b68b7ce78962a895a304528c97d098cc65
Rebased source (tag verify/rebased/20260519T013515Z)	e4d81e48c913	5c448ac374ef292d8333d48b293fcdbd9a228a21

Inspect the difference:

git fetch upstream 1e5ab7e2667c7ccf3ae9059770d1fcd1f3a3fae1
git diff e4d81e48c913 1e5ab7e2667c
git diff e4d81e48c913 1e5ab7e2667c -- <path>  # per-file

Tag verify/rebased/20260519T013515Z preserved for reproduction.

Generated by single_commit_pr_chain.py verify-rebased.

[fzyzcjy]

✅ verify-rebased: PASS

Rebased source tree matches the current main tree (chain has been fully merged; the recorded PR head SHA is the pre-merge snapshot, now stale — that's expected and fine).

	SHA	Tree
Rebased source (tag verify/rebased/20260519T013859Z)	c2a212bfe222	f8f47cec5a620aac42e07d0d07f9b282048ef2d3
PR head (tom/pr_chain/tom_refactor_202605a/primary/nonmech_scheduler/compute-max-prefix-len)	1e5ab7e2667c	975b53b68b7ce78962a895a304528c97d098cc65
upstream/main	c2a212bfe222	f8f47cec5a620aac42e07d0d07f9b282048ef2d3

Reproduce locally (the rebase tag persists after this run):

git fetch upstream 1e5ab7e2667c7ccf3ae9059770d1fcd1f3a3fae1
REB_TREE=$(git rev-parse 'verify/rebased/20260519T013859Z^{tree}')
PR_TREE=$(git rev-parse '1e5ab7e2667c7ccf3ae9059770d1fcd1f3a3fae1^{tree}')
MAIN_TREE=$(git rev-parse 'upstream/main^{tree}')
echo "REB_TREE  = $REB_TREE"
echo "PR_TREE   = $PR_TREE"
echo "MAIN_TREE = $MAIN_TREE"

Generated by single_commit_pr_chain.py verify-rebased.