feat(cli): add generate-types subcommand

[gsmith85]

Summary

Implements Task 4 (PR 1d) of the two-layer ucp-schema code-generation pipeline: wires the ucp-schema generate-types CLI subcommand in src/bin/ucp-schema.rs and adds end-to-end CLI integration tests in tests/generate_types_test.rs.

Changes

• CLI Subcommand (src/bin/ucp-schema.rs):
  • Adds Commands::GenerateTypes (#[command(name = "generate-types")]) with --profile (-p), --capability (--capabilities, comma-delimited, conflicts with --profile), --extension (--extensions, comma-delimited, conflicts with --profile), --schema-dir (-s, alias --schema-local-base), --schema-remote-base (requires --schema-dir), --output (-o), and --pretty (defaults to true).
  • Implements CliExitCode for CodegenError and generalizes write_json_output to <T: serde::Serialize + ?Sized>.
• CLI Integration Tests (tests/generate_types_test.rs):
  • Verifies stdout pretty/compact JSON output, --output <file> generation, repeatable and comma-separated --capability / --extension flags, --schema-local-base alias, and expected CLI exit codes (0, 2, 3).

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 (feat(codegen): add capability reachability, composition, and generate_types() API #83): Capability/extension reachability, composition & generate_types() library API (src/codegen/{mod,reachability,hoist,compose}.rs)
4. PR 1d (this PR): 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: OpenAPI 3.1 REST service & type binding (ucp-schema generate-openapi, src/openapi/mod.rs)

Test Plan

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