Add the embedding surface of the new retrieval-provider capability: an
EmbeddingProvider protocol, the EmbeddingResponse / EmbeddingUsage /
EmbeddingRuntimeConfig types, and an OpenAIEmbeddingProvider reference
impl posting to an OpenAI-compatible /v1/embeddings. Typed
EmbeddingEvent / EmbeddingFailedEvent join the observer event union,
dispatched on every embed(); the bundled OTel and Langfuse observers
safely skip them for now (the embedding span and Embedding observation
are a follow-up).

The provider rejects malformed responses (missing, empty, or non-numeric
vectors and non-permutation indices), guards a trailing /v1 on base_url,
and offers a configurable readiness probe defaulting to a universal
one-input embed so it works against backends without a /v1/models
catalog (e.g. TEI's OpenAI surface). Conformance fixtures 001-005 pass;
unit tests cover the guard, the readiness modes, and the observer
safe-handling.

conformance.toml keeps 0059 not-yet; the observability rendering
(074-083), the cross-provider helper cleanups, and the docs are v0.16.0
follow-ups.

Fix the issues from the PR #196 re-review:

- Build the embeddings request body extras-first so a caller's
  undeclared runtime-config extra cannot clobber the bound model or the
  input list.
- Reject non-numeric vector values (JSON strings, bools) and bool usage
  counts strictly rather than coercing them, so a non-numeric vector is
  treated as malformed.
- Dedup the observer event union: correlation and both observers now
  reference the single ObserverEvent alias instead of repeating the
  member list, so a new event widens the contract in one place.
- Move a stray spec section ref out of a docstring into a comment and
  reword em-dash substitutes in docstrings.
- Strengthen the tests: the Langfuse safe-handling test asserts no trace
  is created, and the conformance runner tolerates a fixture without a
  stores_response_in key instead of raising KeyError.
- Refresh the observer dispatch docstrings to point at ObserverEvent
  rather than a hardcoded variant count.

From the PR #196 review:

- _probe_models now raises ProviderInvalidResponse when the /v1/models
  body is not a JSON object or its data field is missing or not a list,
  mirroring _parse_response. A malformed catalog was being degraded to an
  empty list and misreported as a missing model (ProviderInvalidModel);
  that category is now reserved for a well-formed catalog that genuinely
  lacks the bound model. Covered by a new unit test.
- Align the _classify_embedding_http_error docstring with the code: the
  catch-all maps every other status (not just 5xx) to unavailable, the
  same as the sibling classify_http_error.

The Observer protocol docstring example annotated its log_observer with
NodeEvent | MetadataAugmentationEvent, which would be a structural
conformance error now that the protocol delivers the full ObserverEvent
union (embedding, tool, and provider events included). Annotate the
example with ObserverEvent so copied observers type-check.