feat(extension): manifest-less lfx.bundles discovery + precedence tier

[erichare]

Bundle Separation Phase A — PR 1: foundation

Part of Bundle Separation Phase A — metapackage split (1.11) (follow-on to LE-905). lfx becomes engine-only; the ~78-provider long tail collapses into one manifest-less lfx-bundles metapackage discovered via a new lfx.bundles entry-point; ~5 partners graduate to standalone lfx-<provider>. Zero user-facing change. This PR is the foundation the rest of the stack builds on.

What this does

Adds a manifest-less lfx.bundles discovery path + a new precedence tier, extending the existing extension/bundle discovery (not rewriting it).

• New src/lfx/src/lfx/extension/loader/_bundles_root.py::load_lfx_bundles_extensions — iterates the lfx.bundles setuptools entry-point group, resolves each declared package via find_spec, folder-walks its immediate subdirectories, and registers each as a bundle at the @official slot with no extension.json (the langchain-community model). Mirrors the existing discover_inline_bundles manifest-less walk and reuses _load_bundle_directory.
• Discovery precedence is now installed > seed > lfx_bundles > dev > inline (_DISCOVERY_PRECEDENCE in interface/components.py). Manifest always wins, so a manifest-shipping lfx-<provider> shadows the same-named provider in the metapackage with a typed bundle-shadowed warning — this is what lets a provider graduate with no lockstep release.
• New bundle-discovery-malformed error code, emitted on warnings (never flips ok / aborts startup) for unresolvable declarations and invalid provider folder names.
• Exported via loader/__init__ and the lfx.extension PEP-562 lazy surface.

Test plan

• 10 new unit tests in test_load_lfx_bundles.py (discovery + @official registration, invalid-name + malformed-declaration warnings, manifest-shadows-manifestless via _resolve_bundle_shadowing)
• Full extension unit suite green (449 passed) — incl. test_components_cache_integration which exercises the rewired import_extension_components
• ruff + ruff format clean; mypy clean on the new module

Stack (merge bottom-up)

1. bundles/foundation-discovery → feat/bundle-metapackage-split ← this PR
2. bundles/create-lfx-bundles → bundles/foundation-discovery (skeleton)
3. bundles/lfx-engine-only → bundles/create-lfx-bundles (engine-only)

Merges into the integration branch feat/bundle-metapackage-split; the tracking/meta PR to release-1.10.0 opens once the stack lands. Draft until QA signs off on this ticket.

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