Retry marker assertions after stale marker

[TyceHerrman]

What does this PR do?

Retries WorkspaceHelpers.assertMarker(...) once with a fresh marker snapshot when a marker disappears during inspection. Unrelated CoreExceptions continue to propagate, and the existing missing-marker diagnostics are preserved.

Adds focused coverage for stable markers, stale-marker replacement, bounded retry, unrelated failures, and missing-marker diagnostics.

Fixes #2224

How was this tested?

mvn -U -B -ntp clean integration-test -am -pl org.eclipse.m2e.core.tests -Dtest=WorkspaceHelpersTest -Dsurefire.failIfNoSpecifiedTests=false -P=-eclipse-sign-jnilibs

Result: 5 tests, 0 failures, 0 errors, 0 skipped; all 10 reactor modules succeeded.

AI/GenAI disclosure

• Tool/agent + model/version + mode: Codex, GPT-5, interactive
• I have personally reviewed all code, tests, and text in this PR and understand and stand behind every change, per the Eclipse GenAI contribution guidelines.
• I will personally review and confirm any AI-assisted follow-up changes made in response to review comments before pushing them (see AGENTS.md rule on reading full thread history).

Checklist

• I have searched existing PRs/issues and this is not a duplicate.
• mvn clean verify (add -Pits for integration tests) passes locally.
• Bundle versions are bumped where required (see "Version bump" in CONTRIBUTING.md).
• I have signed the Eclipse Contributor Agreement (ECA).

🤖 Drafted with Codex, GPT-5, interactive. Reviewed and confirmed by @TyceHerrman before submission.