Avoid merging computed dependency metadata

[floze-the-genius]

Summary

• discard computed dependency fields from base services before resolving extends
• prevent _dependents sets from reaching the list merge path
• add a regression test covering extended services that both have dependents
• add the required bugfix news fragment

Fixes #1507.

The implementation was prepared with OpenAI Codex and manually reviewed against the dependency-flattening and extends-resolution flow.

Tests

• python -m unittest -v tests.unit.test_depends_on tests.unit.test_rec_merge_depends_on - 15 passed
• ruff format --check podman_compose.py tests/unit/test_rec_merge_depends_on.py - passed
• ruff check podman_compose.py tests/unit/test_rec_merge_depends_on.py - passed
• git diff --check - passed

mypy . still reaches repository/tooling baseline issues (unsupported configured Python 3.9, missing PyYAML stubs, and an existing error at podman_compose.py:3274). Integration tests requiring Podman were not run locally.

[floze-the-genius]

@p12tic A second fix for #1507 (#1509) was opened shortly after this PR, so a quick direction would help avoid duplicate work. This implementation treats both _deps and _dependents as computed DependField metadata, strips them before extends merging, and verifies that _dependents is rebuilt for the child. Current head 94c8303 is mergeable and all 33 checks, including DCO, pass; the required unit test and newsfragment are present. Would you prefer this approach or the narrower _dependents-only change in #1509? I can rebase the selected branch.