fix(ci): preserve next README prose in release policy

[ankitTelnyx]

Summary

• Preserve next README prose during both synthetic Release Please scanning and final release-tree replacement. Copy only the recognized Maven badge link, Gradle dependency, and Maven dependency versions.
• Extend the existing production-owned copy_gradle_version.py with explicit --readme mode; existing Gradle callers remain unchanged. Missing, duplicate, malformed, inconsistent or unmarked version slots fail before target writes.
• Regression executes the actual workflow shell fragments in a temporary Git repository, checks index and worktree prose, and covers arbitrary non-version content, Unicode/CRLF and invalid source/target shapes.

Verification

• RED: original workflow regression failed because the complete added MCP/prose section disappeared.
• GREEN: python3 -m unittest discover -s .github/scripts -v — 76 tests passed.
• ruff format --check on both Python files; git diff --check.
• actionlint passes with only the existing SC2034 unused attempt warning ignored; the identical warning was reproduced against unmodified master.
• Actual fetched origin/next README overlay retained MCP and exact non-version parity.
• SDK Java source/build files unchanged; no Gradle build or publication performed.

Ownership / durability / rollout

This is production-owned release policy, so the base is master, not staging. No new repository paths. Inspected live team-telnyx/telnyx-java-staging/.github/workflows/promote-to-prod.yml: all three changed paths already restore from production master in both independent release-policy synchronization and full promotion. The existing release policy CI already invokes test_copy_gradle_version.py.

After explicit approval/merge, run existing staging release-policy synchronization to carry this fix to production next and regenerate the open release PR, then verify its exact-head README and CI. This PR does not patch the generated release branch, merge anything, or publish a release.

Local durable plan: clawd/.hermes/plans/java-readme-policy-v1.md (v1). Independent plan/diff review remains pending parent/human review; draft intentionally retained. Hosted CI is pending at creation.