Harden the orphan-fallback conformance driver and the handler-enumeration guard

[chris-colinsky]

Blocks v0.17.0. Test-quality items from two adversarial reviews of #277. None changes shipped behaviour; each is a case where a green run means less than it appears.

1. _handler_event_types() silently drops handlers

tests/unit/test_observability_otel.py derives its coverage list by regexing the observer's source for def _handle_*(self, event: ...) followed by event=event,. That misses a multi-line signature, a differently-named parameter, and a call site passing the event positionally — so a future handler opting into orphan-path synthesis is silently uncovered, which is exactly the class the guard exists to close.

Prefer inspect.signature over a regex, and assert the parsed set is non-empty and matches an expected count, so a parse that silently narrows fails loudly.

2. test_dispatch_span_openers_tolerate_every_optional_field_being_absent never calls an opener

It drives _event_caller_metadata / _apply_caller_metadata / _subgraph_identity_at directly. Confirmed by mutation: reverting the openers' defensive read leaves it green — only the end-to-end test_failure_isolated_marker_survives_orphan_path_synthesis fails. Either drive a real opener or rename it to say it tests the helpers.

3. _stored_lineage does not guard chain_len == 0

branch_name_chain[: chain_len - 1] becomes [:-1] at chain_len == 0, which drops the last element instead of returning empty. Not currently reachable (a dispatch span always has a non-empty prefix), so it is a latent trap rather than a live defect.

4. Conformance driver nits (tests/conformance/test_observability.py)

• _mock_bodies_by_request_content validates the pairing with a count over a list but returns a dict keyed by request content, so two wrappers declaring the same content collapse to one entry while the count assertion still passes — making orphan_llm_span_routed_to_correct_branch_by_name vacuous.
• The mock _handler's AssertionError on unmatched content is raised inside fan-out instance middleware under error_policy: collect, which absorbs it, so a pairing failure surfaces as an unrelated missing-span message rather than the handler's diagnostic.
• dispatch_spans_close_in_declaration_order reads end times from a dict whose insertion order is event-arrival order, which Resolve orphan provider span parents structurally, wire 152/153 #277 makes depend on which branch's wrapper-issued event drains first.