refactor(backends): move generate_from_raw hook firing into Backend base class

[ajbozarth]

Pull Request

Issue

Fixes #1183. Folds the additive half of #1218 Part 1.

Description

Brings the raw (batch) generation path in line with the chat path's
existing wrapper pattern: Backend.generate_from_raw becomes a @final
wrapper that owns hook firing, and backends implement only the new
_generate_from_raw abstract. The three generation_batch_* hooks now
fire from one place instead of being duplicated inline in all five
backends.

Reviewer call-outs:

1. Tuple return on _generate_from_raw. Backends return
   tuple[list[ModelOutputThunk], dict | None] — (results, usage). The
   wrapper unpacks the tuple, fires post_call with the aggregate, and
   returns just results to callers (public signature unchanged).
   Considered moving usage aggregation into the wrapper, but openai and
   litellm only report whole-batch usage at the response root, so the
   impl is the only place that knows the right shape. Matches the
   existing _generate_from_context -> tuple[MOT, Context] precedent.

2. Standardized self._model_id and self._provider instead of new
   abstracts. The wrapper needs model and provider for hook
   payloads (pre_call has no MOT yet; error may have no MOT). Considered
   adding _provider_name / _resolved_model_id() abstract members.
   Instead, finished the partial convention 3 of 5 backends already
   used: every backend now sets self._model_id: str and
   self._provider: str in __init__. Inlines model-id resolution into
   ollama and huggingface init bodies and deletes the now-unused
   _get_*_model_id helpers. Renames LocalHFBackend._hf_model_id to
   _model_id for consistency. ~10 chat-path setter sites switch to
   reading these attributes — no value changes, just one canonical
   source per backend.

3. BaseException on the raw-path error wrapper, matching chat. The
   chat-path wrapper's BaseException was added in refactor(telemetry)!: move backend tracing onto plugin/hook pattern #1181; the raw path
   was tentatively left at except Exception. The same gap exists:
   synchronous KeyboardInterrupt / asyncio.CancelledError /
   SystemExit inside the impl currently bypass the error hook
   silently. This PR aligns to BaseException. Behavior change: raw
   cancellation/interrupts now fire generation_batch_error before
   propagating.

4. Batch pre_call payload mutations now propagate. When the batch
   hooks were added in fix(backends): unify raw-path token usage on mot.generation.usage; guard eval_count=None #1218 they were wired for telemetry-only
   observation: a plugin could mutate model_options / format /
   tool_calls on the pre_call payload, and the chat path would honor
   those mutations, but the batch path silently dropped them. Same
   plugin, same intent, different result depending on which API the
   user reached. The wrapper now captures the post-hook payload and
   reassigns the locals before calling _generate_from_raw, identical
   to the chat-path idiom. Adds model_options to
   GenerationBatchPreCallPayload (it didn't exist on the batch
   payload at all). Test inverted from _are_not_propagated to
   _propagates, mirroring the chat-path test.

5. Folded the additive half of fix(backends): unify raw-path token usage on mot.generation.usage; guard eval_count=None #1218 Part 1. Backends with per-MOT
   token counts (ollama, huggingface, watsonx) now also populate
   mot.generation.usage per MOT. Backends with whole-batch-only usage
   (openai, litellm) leave per-MOT mot.generation.usage = None and
   surface the aggregate via the tuple return. Null-token policy:
   all-or-nothing — if any of prompt_tokens / completion_tokens /
   total_tokens cannot be determined for a MOT, that MOT's
   generation.usage stays None (matches dict | None typing; ollama
   docs document Optional[int] as "not yet available" rather than
   "zero"). Watsonx's previous undocumented or 0 coerce switches to
   the same policy.

   The existing mot._meta["usage"] writes are preserved everywhere —
   budget_forcing_alg.py still reads from them. fix(backends): unify raw-path token usage on mot.generation.usage; guard eval_count=None #1218's remaining
   scope (consumer-side reads in budget_forcing_alg.py and the
   mot._meta["usage"] deprecation path) stays open for follow-up.

   Originally folded in to support generic usage aggregation in the
   wrapper; kept after the openai/litellm pivot to call-out 1's tuple
   return because the per-MOT writes are still the right thing for the
   3 backends whose APIs expose them.

6. Custom-backend doc updated.
   docs/docs/community/building-extensions.md was teaching users to
   override the public generate_from_context / generate_from_raw
   directly; those are @final now. The example was updated to
   implement _generate_from_context / _generate_from_raw and set
   _model_id / _provider.

7. Test coverage. Adds TestGenerationBatchHookCallSites in
   test/plugins/test_hook_call_sites.py mirroring the chat-path
   firing-site tests (9 tests). _MockBackend extended to cover both
   paths with hardcoded behavior; error case via inline subclass per
   the existing RecordingBackend(_MockBackend) pattern. Mocks in
   test/stdlib/test_streaming.py, test/core/test_logger_plugin_hooks.py,
   and test/stdlib/frameworks/test_react_framework.py updated to the
   new abstract contract.

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Attribution

• AI coding assistants used

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