Restore missing-entry guard in CoursierResolvedLockfile.dependencies() (regression from #22906) (Cherry-pick of #23341)

[WorkerPants]

Disclaimer: I used Claude Code to evaluate and confirm comments from #23328. Basically, part of #22906 needs to be reverted.

Okay, now here's Claude:

Summary

Fixes a KeyError regression introduced by #22906 that breaks pants check (and other JVM goals) on lockfiles whose entries reference a transitive dependency that has no top-level coord entry of its own.

#22906 bumped Coursier to v2.1.24 to pick up the fix for coursier#2884 and, on the assumption that the only remaining "missing entry" cases would be parent POMs (classifier="pom"), simplified CoursierResolvedLockfile.dependencies() to do a direct dict lookup instead of an entries.get(...) guard. That assumption turns out to be wrong: even fresh resolves with the latest Coursier (verified against v2.1.25-M19) still emit transitive deps without a coord entry — e.g. ("org.apache.curator", "apache-curator", None) when resolving org.apache.hive:hive-exec:1.1.0, or ("org.apache.arrow", "arrow-memory", None) in the user-reported repro on hive-exec:3.1.3. Both have classifier=None, so the if d.classifier != "pom" filter doesn't catch them, and the unguarded lookup raises KeyError.

This restores the pre-#22906 entries.get(...) skip and adds the bug-report repro as a regression test.

Changes

• src/python/pants/jvm/resolve/coursier_fetch.py: restore the entries.get(...) guard in dependencies(); update the comment to reflect that [trigger ci] some ivy work in progress #2884-style missing entries are still observable on current Coursier.
• src/python/pants/jvm/resolve/coursier_fetch_test.py: add test_dependencies_skips_transitive_entries_missing_from_lockfile — the exact fixture from the bug report. Without the fix it reproduces KeyError: ('org.apache.arrow', 'arrow-memory', None); with the fix it passes.

Out of scope

• CoursierResolvedLockfile.direct_dependencies() has the same unguarded pattern but predates Update Coursier default version to v2.1.24 #22906; leaving for a follow-up PR.
• The longer-term transitive-closure rewrite drafted in plans/20251117_jvm_transitive_dep_bug.md is also a follow-up.

Test plan

• New unit test test_dependencies_skips_transitive_entries_missing_from_lockfile passes with the fix and fails (with the original KeyError) without it.
• ./pants test src/python/pants/jvm/resolve/coursier_fetch_test.py src/python/pants/jvm/resolve/coursier_fetch_integration_test.py — all pass.
• ./pants fmt lint check clean on both changed files.