fix: avoid duplicate needs from nested docs bundles

[AlexanderLanin]

Summary

• expose only direct bundle sources as Sphinx top-level sources while keeping transitive files available for mounts
• add producer/consumer docs_bzl coverage for a docs_bundle in a subdirectory
• verify public docs() targets and prevent duplicate needs in needs_json

Bug

When a docs() target included a docs_bundle from a subdirectory, the complete transitive bundle DefaultInfo was passed as Sphinx srcs. Nested sources were then processed once as raw sources and once through score_mounts, causing needs.duplicate_id failures when the docs bundle was consumed by another docs() target.

Required for eclipse-score/score#3215

Validation

• targeted subdirectory_bundle scenario: 3 passed
• public docs() target conflict check: passed
• existing nested, external, and basic docs_bzl tests: passed
• sandboxed data-bundle needs_json build: passed
• commit hooks: Ruff, BasedPyright, and copyright checks passed

Note: the existing data_files_runfiles runtime smoke test can encounter workspace-wide test-log warnings in a dirty workspace; its needs_json build passes.

[github-actions]

License Check Results

🚀 The license check job ran with the Bazel command:

bazel run --lockfile_mode=error //src:license-check

Status: ⚠️ Needs Review

Click to expand output

[License Check Output]
Extracting Bazel installation...
Starting local Bazel server (8.6.0) and connecting to it...
INFO: Invocation ID: 5fe72127-295c-4420-b070-0e2a3123ea7c
Computing main repo mapping: 
Loading: 
Loading: 0 packages loaded
Loading: 0 packages loaded
Loading: 0 packages loaded
    currently loading: src
WARNING: Target pattern parsing failed.
ERROR: Skipping '//src:license-check': no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
ERROR: no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
INFO: Elapsed time: 5.693s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target

[github-actions]

Documentation preview for this pull request is available at:
pr-758: https://eclipse-score.github.io/docs-as-code/pr-758/

[MaximilianSoerenPollak]

Looked through in a call. Looks alright 💯

[MaximilianSoerenPollak]

Talked about in a call. Looks good 💯