Add support for legacy APT clients that do not fetch binary-all - #1521

Fixes #447.

This adds an opt-in no_support_for_architecture_all setting to APT
publications for compatibility with APT clients before version 1.1.

When enabled, structured publications:

• retain the binary-all/Packages index;
• include each Architecture: all package in every architecture-specific
  Packages index; and
• add No-Support-for-Architecture-all: Packages to generated Release
  metadata.

The setting defaults to disabled, preserving the existing publication format.
The API field is stored on the publication so consumers can determine how a
given publication was generated.

Testing

• python3 -m compileall -q pulp_deb
• ruff format --check
• ruff check
• git diff --check
• uv build --wheel
• oci-env pulpcore-manager makemigrations deb --check --dry-run
• oci-env test -p pulp_deb lint
• oci-env test -p pulp_deb unit — 65 passed, 2 skipped
• Focused publication tests — 2 passed
• Signing functional tests — 10 passed
• Full functional suite collection — 153 tests collected successfully
• The broad functional suite and its three upstream-CI-setting-dependent cases
  were exercised locally; all three cases passed after matching the official
  APT_BY_HASH and legacy-checksum settings
• Ubuntu 12.04 and 14.04 APT smoke tests discovered and downloaded an
  Architecture: all package through binary-amd64/Packages

📜 Checklist

• Commits are cleanly separated with meaningful messages
• A changelog entry has been added
• Follows the Pulp policy on AI Usage
• User documentation and test coverage have been added