feat(http): implement ADR-051 YAML input for from_openapi — from_yaml + JSON-first from_str

Adds OpenAPISpec::from_yaml and OpenAPISpec::from_str alongside the
existing from_json. from_str is JSON-first, YAML-fallback (ADR-051 §2):
a valid JSON doc parses through serde_json (stricter grammar, immune to
YAML-specific coercion); only on JSON failure does it fall back to the
YAML parser. yaml_serde 0.10 (resolves to 0.10.4, latest stable) is the
maintained official-YAML-org fork of the deprecated serde_yaml; not
feature-gated per ADR-051 §3 (YAML OpenAPI is first-class).

Note: ADR-051 §2's stated YAML 1.1 boolean-coercion rationale does not
hold for yaml_serde 0.10 (YAML 1.2 core schema — yes/no/on/off are
strings). The JSON-first rule is retained per the Accepted ADR as a
defensive default; tests document the actual 0.10.x behavior with
explicit comments. The ADR rationale should be amended separately.

256 tests pass; clippy + fmt clean.

tasks/http/adapters/from-openapi-yaml-input.md

@@ -1,7 +1,7 @@
 ---
 id: http/adapters/from-openapi-yaml-input
 name: Add YAML input format to from_openapi (from_yaml + format-detecting from_str, JSON-first per ADR-051)
-status: pending
+status: completed
 depends_on: []
 scope: narrow
 risk: low