refactor(adapter): simplify base adapter flow

[isaacbmiller]

Summary

This is PR 2 of 2 in the adapter simplification stack. It sits on top of #63.

This PR simplifies dspy/adapters/base.py now that PR 1 gives native response types a normalized LMOutput parse hook. The adapter still builds an LMRequest internally, but it does not change dspy.LM, BaseLM, or the provider/client boundary.

Key changes:

• Collapses the adapter call path into a small prepare/call/postprocess flow.
• Keeps the current BaseLM boundary by converting the internal LMRequest back to legacy OpenAI-shaped kwargs inside the adapter edge.
• Parses normalized LMResponse / LMOutput values directly instead of converting back through legacy postprocess dictionaries.
• Preserves native tool-call parsing, ToolCalls output filling, native response types, logprobs, and empty-output handling.
• Keeps custom/unknown legacy content blocks round-trippable while adapters still render legacy Type.format() content blocks.

Why This Belongs After #63

The base adapter can only parse LMResponse directly once custom/native response fields know how to read normalized output parts. #63 adds that output hook and the small history/type bridge; this PR then uses it to remove the bulk of the internal legacy parsing path.

Review Notes

This deliberately avoids the LM/client files. The adapter builds LMRequest because that is the internal shape we want to move toward, but the call into current BaseLM remains:

1. Build normalized LMRequest from adapter-rendered messages.
2. Convert it to current BaseLM-compatible kwargs with to_openai_chat_request(...) at the adapter edge.
3. Call lm(messages=..., **kwargs) / lm.acall(...).
4. Normalize legacy LM outputs to LMResponse for adapter parsing.

That leaves the future deletion path clear: when BaseLM accepts/returns normalized request/response objects, remove the adapter-local _legacy_call_kwargs(...) and legacy-output normalization call.

Validation

• uv run --frozen ruff check dspy/adapters/base.py dspy/adapters/types/base_type.py dspy/adapters/types/citation.py dspy/adapters/types/reasoning.py dspy/core/types.py
• uv run --frozen pytest --deno -q tests/core/test_types.py tests/streaming/test_streaming.py::test_streaming_allows_custom_streamable_type tests/adapters tests/signatures/test_adapter_file.py
  • 279 passed, 14 warnings
• uv run --frozen pytest --deno -q tests
  • 1046 passed, 233 skipped, 2 xfailed, 50 warnings