Issue #601: consolidate LLM client registry resolution

[stranske]

Source: Issue #601

Closes #601

Summary

• move LangChain slot/registry resolution into tools.llm_registry as the single source of truth
• keep tools.langchain_client focused on LangChain object construction with compatibility wrappers
• add regression tests that fail if local slot/registry resolver classes are reintroduced or the script adapter stops delegating

Validation

• UV_CACHE_DIR=/tmp/pd-workloop-uv-cache uv run ruff check tools/langchain_client.py tools/llm_registry.py scripts/langchain/_llm_client.py tests/tools/test_langchain_client_config.py tests/tools/test_llm_client_single_source.py
• UV_CACHE_DIR=/tmp/pd-workloop-uv-cache uv run pytest tests/tools/test_langchain_client_config.py tests/tools/test_llm_client_single_source.py -q --no-cov
• Deliberate break: temporarily reintroduced SlotDefinition in tools/langchain_client.py; test_langchain_client_uses_llm_registry_as_single_resolution_source failed, then the break was reverted and tests passed again.

Summary by CodeRabbit

• Refactor

  • Centralized LLM model/slot resolution, including slot config loading with sensible defaults, per-slot environment overrides, and blocked-model filtering for safer provider/model selection.

• Tests

  • Added coverage to ensure model/slot resolution uses a single source of truth and that the client adapter delegates client creation correctly.
  • Added configuration tests verifying invalid slot payload shapes (non-list and non-object entries) are ignored and default slots are used.

Automated Status Summary

Scope

Scope section missing from source issue.

Context for Agent

Related Issues/PRs

• #596

Tasks

• Choose the canonical module (likely tools/llm_provider.py), migrate all callers to it, and remove the redundant module(s).
• Route provider/model resolution — including loading config/model_registry.json + config/llm_slots.json (see [P1] config/model_registry.json + config/llm_slots.json are never loaded by any src/ module (inert LLM-routing config) #596) — through the one module.

Acceptance criteria

• Named test: provider resolution for a given slot goes through the canonical module and honors the registry. Deliberate-break -> revert: re-add a second resolver path a caller uses -> a "single source of truth" assertion (or the registry test) FAILS -> revert.
• grep -rl <removed module> returns no callers after migration.