fix: drop unmentioned related columns from values()/values_list()

[collerek]

Fixes #800.

values() / values_list() were leaking columns of related models that got pulled into the query by a filter (e.g. filter(project__id=...)) or a bare select_related() but were never named in fields(). The flat-projection result included every column of the joined model on top of what the caller actually requested.

The fix adds ExcludableItems.with_projection_exclusions(source_model, select_related), called from QuerySet.values(). When fields() was used, it walks each select_related path and adds an implicit exclude for any segment (and its m2m through model) the user never referenced. Regular ORM-load queries (get, all, etc.) are unchanged - they still load all columns of unmentioned related models so Pydantic construction does not lose mandatory fields.

[codspeed-hq]

Merging this PR will degrade performance by 29.25%

⚡ 3 improved benchmarks
❌ 1 regressed benchmark
✅ 94 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_creating_individually_with_related_models[40]	354.2 ms	277.9 ms	+27.42%
⚡	WallTime	test_get_or_create_when_create[10]	119 ms	99.6 ms	+19.56%
⚡	WallTime	test_saving_models_individually_with_related_models[20]	176.3 ms	140.7 ms	+25.36%
❌	WallTime	test_saving_models_individually_with_related_models[40]	248.5 ms	351.3 ms	-29.25%

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_{Comparing fix-issue-800-values-list-fields-leak (e5d975e) with master (5c86ce1)}