feat(codegen): add capability reachability, composition, and generate_types() API

[gsmith85]

Summary

Implements Task 3 (PR 1c) of the two-layer ucp-schema code-generation pipeline: capability/extension reachability discovery, $defs hoisting & collision qualification, extension composition, and the public compile_types() / generate_types() library API.

Pipeline Stages Added

• Stage 1 (src/codegen/reachability.rs):
  • Discovers all .json schemas in --schema-dir using schema structure (name/version capability identity, extends metadata, ucp_request/ucp_response directional annotations, and $defs reverse-domain parent inference), supports short-name (checkout) and reverse-domain (dev.ucp.shopping.checkout) matching, reclassifies extension schemas passed via --capability, seeds ambient protocol roots (profile, ucp, capability, service, payment_handler, error_response), and computes the transitive $ref closure while pruning inactive extension $defs.
• Stage 2 (src/codegen/hoist.rs):
  • Classifies and hoists $defs entries into the top-level $defs bundle, skips pure re-exports, extracts self-named capability role schemas (*PlatformSchema, *BusinessSchema, *ResponseSchema), and parent-qualifies colliding local $defs (PaymentAp2MandateErrorCode, JsonrpcErrorResponse, A2aMessageMessage).
• Stage 3 (src/codegen/compose.rs):
  • Composes active extensions into root capabilities (resolving root definitions via schema structure or self-named definitions), in-place standalone sub-type overlays (Buyer, Payment, PaymentInstrument, Cart -> Checkout), and container capability operations (CatalogSearch*, CatalogLookup*, supporting both nested $defs and oneOf/allOf extension shapes).
• Stages 5, 6, 8 & Public API (src/codegen/mod.rs, src/lib.rs):
  • Orchestrates directional slicing, $ref alignment, and empty-object pruning (retaining ErrorResponse and definitions referenced via $ref anywhere in $defs or root properties), and exposes GenerateTypesOptions, TypesBundleDoc, CompiledTypes, CodegenError, compile_types(), and generate_types().

Stacked PR Series Roadmap

1. PR 1a (feat(codegen): add identifier naming, UCP keyword stripping, and $defs ref rewriting #81): Identifier qualification, UCP keyword stripping, $ref rewriting (src/codegen/normalizer.rs)
2. PR 1b (feat(codegen): add directional schema slicing and $ref alignment #82): Directional schema slicing & $ref alignment (src/codegen/normalizer.rs)
3. PR 1c (this PR): Capability/extension reachability, composition & generate_types() library API (src/codegen/{mod,reachability,hoist,compose}.rs)
4. PR 1d (feat(cli): add generate-types subcommand #84): CLI generate-types subcommand (src/bin/ucp-schema.rs, tests/generate_types_test.rs)
5. PR 2 (feat(codegen): add self-contained .well-known/ucp profile scoping for generate-types #85): Self-contained .well-known/ucp --profile scoping (src/codegen/profile.rs)
6. PR 3a (feat(codegen): add AST shape normalizers and inline conditional variant hoisting #86): Codegen AST shape normalizers & inline conditional variant hoisting (src/codegen/normalizer.rs, src/codegen/reify.rs)
7. PR 3b (feat(codegen): lower open string discriminator unions into ordered anyOf with <Union>Base #87): Open string discriminator union lowering (<Union>Base) (src/codegen/reify.rs)
8. PR 3c (feat(codegen): discover and register decentralized extended subtypes into open unions #88): Cross-file extended subtype discovery & registration (src/codegen/reify.rs)
9. PR 4a (python-sdk#101): Python SDK direct JSON Schema codegen (generate_models.sh, postprocess_models.py, src/ucp_sdk/models/)
10. PR 4b (js-sdk#87): JS/TS SDK direct JSON Schema codegen (generate_models.sh, scripts/generate-zod-from-types.mjs, src/spec_generated.ts)
11. PR 5 (feat(openapi): add profile-driven OpenAPI 3.1 service and type binding (generate-openapi) #89): OpenAPI 3.1 REST service & type binding (ucp-schema generate-openapi, src/openapi/mod.rs)

Test Plan

• cargo fmt --check
• cargo test --all-targets
• cargo clippy --all-targets -- -D warnings