models: update_custom + delete_custom, fix add/remove filter

[stepdi]

Summary

• New update_custom(model_id, *, api_url, api_key, max_tokens) (sync + async) — repoint a custom model's mutable fields without recreating it. Customer's primary workflow: ephemeral vLLM endpoints behind cloudflared tunnels whose URL rotates between sessions.
• New delete_custom(model_id) (sync + async) — full teardown that disables the record, strips it from Project.Models, releases the name for reuse.
• Fix add() / remove() to operate on the full project model list (public + custom). The previous type="public" filter silently dropped custom-model IDs from Project.Models on every call.

Companion to atlas-app PR https://github.com/LayerLens/atlas-app/pull/1916, which lands the matching backend routes (PATCH + DELETE /custom-models/{id} on the API-key router) and the PatchProject validator fix that the SDK depends on. Merge atlas-app first, then this one.

Customer context

client.models.remove(custom_model_id) was returning a 500 referencing a different model. Root cause was the atlas-app PatchProject validator only checking the public ModelsCollection; the new update_custom path lets the customer repoint their tunnel-backed model in place, which is what they actually wanted to do all along. delete_custom is the explicit teardown path for the rare cases where recreate is needed.

What changed

• src/layerlens/resources/models/models.py — three new flows (sync + async); docstring + behavior change on add / remove.
• tests/resources/test_models_resource.py — 12 new unit tests covering update_custom (partial / full / error / non-dict response), delete_custom (happy / error envelope), plus regression tests that add/remove now preserve custom models in the PATCH payload.
• tests/test_models_custom_live.py — new @pytest.mark.live end-to-end (create → update → delete → confirm gone). Auto-skipped without LAYERLENS_STRATIX_API_KEY. Includes best-effort cleanup on failure.
• tests/test_samples_e2e.py — mocked client fixture grew update_custom / delete_custom stubs.
• docs/api-reference/models-benchmarks.md — new update_custom and delete_custom sections; add / remove blurbs reworded.
• docs/examples/models-and-benchmarks.md — two worked examples (repointing api_url; replacing a custom model).
• samples/core/custom_model.py — extended to exercise update + delete after the existing create flow.

Test plan

• pytest tests/resources/test_models_resource.py — 68 pass (12 new)
• pytest tests/test_samples_e2e.py -k custom_model — sample runs cleanly under the mocked client
• pytest tests/test_models_custom_live.py — skips cleanly without API key
• mypy src/layerlens/ — 70 files clean
• Live run against staging once atlas-app PR #1916 is deployed
• Customer-confirmed repro: models.remove(custom_id) returns success, and update_custom(custom_id, api_url=...) lands the new URL

Notes

• 24 pre-existing CLI test errors in tests/cli/test_commands.py (CliRunner(mix_stderr=False) is removed in click ≥ 8.2) are unrelated and predate this branch.

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