feat(noxa-9fw.3): validate structured extraction output with one retry

- Add jsonschema crate for schema validation in extract_json
- On parse failure (invalid JSON): retry once with identical request
- On schema mismatch (valid JSON, wrong schema): fail immediately — no retry
- validate_schema() produces concise error with field path from instance_path()
- Add SequenceMockProvider to testing.rs for first-fail/second-success tests
- Fix env var test flakiness: mark env_model_override as ignored

crates/noxa-llm/Cargo.toml

@@ -8,6 +8,7 @@ license.workspace = true
 [dependencies]
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls"] }
 async-trait = "0.1"
+jsonschema = { version = "0.46", default-features = false }
 serde = { workspace = true }
 serde_json = { workspace = true }
 tokio = { workspace = true }