fix(linter): skip the $id check for OpenAPI and OpenRPC service definitions

[yingzhehu-TT]

Summary

• Gate the W002 "schema missing $id" warning on a document-type check.
• Add is_service_definition(): a document declaring a version string under a root
  openapi or openrpc key is a service definition, not a JSON Schema.
• Add three regression tests: OpenAPI skips the warning, OpenRPC skips the warning,
  and a JSON Schema that merely has a property named openapi still warns.

Motivation

lint_file treats every .json file it is handed as a JSON Schema. OpenAPI and
OpenRPC documents are not JSON Schemas, and neither root object defines $id —
so the warning asks authors to add a key their own specification does not permit.

The effect is not cosmetic. On the UCP repository at 7dc6c11:

$ ucp-schema lint --strict source/
✗ 100 files checked: 94 passed, 6 failed (0 errors, 6 warnings)
$ echo $?
1

All six failures are W002, and all six are service definitions:

File	Root keys
handlers/tokenization/openapi.json	openapi, info, paths
services/payment-actions/embedded.openrpc.json	openrpc, info, servers
services/shopping/embedded.openrpc.json	openrpc, info, servers
services/shopping/mcp.openrpc.json	openrpc, info, servers
services/shopping/permalink.openapi.json	openapi, info, servers
services/shopping/rest.openapi.json	openapi, info, servers

Zero genuine JSON Schema files in that tree lack $id. So --strict reports a
100% false-positive rate on the UCP source tree, and the flag cannot be adopted
in CI — the one thing it currently blocks on is a warning that does not apply.

Scope

Only the W002 check is gated. $ref resolution, ucp_* annotation checks,
requires checks and the examples conformance check continue to run on service
definitions unchanged — those documents do carry $refs, and a broken one there
is still a real defect worth reporting.

The detection deliberately requires the root value to be a string. A JSON
Schema that describes an OpenAPI document has an openapi property whose value
is a subschema object; that must keep warning, and the third test pins it.

Validation

• cargo test — full suite passes, including the 3 new tests
  (lint_openapi_service_definition_skips_missing_id_warning,
  lint_openrpc_service_definition_skips_missing_id_warning,
  lint_non_string_openapi_property_still_warns).

• cargo clippy --all-targets -- -D warnings — clean (exit 0).

• cargo fmt --check — clean.

• End-to-end against Universal-Commerce-Protocol/ucp@7dc6c11:

  	ucp-schema lint --strict source/
  before (released 1.4.1)	exit 1 — 100 checked, 94 passed, 6 failed
  after	exit 0 — 100 checked, all passed

• Non-strict ucp-schema lint source/ output is byte-identical before and after
  apart from the removed warnings, confirming no other behaviour changed.

Follow-up (not in this PR)

With this fixed, ucp-schema lint --strict source/ becomes adoptable in the UCP
repo's CI. That is a natural next step for
.github#21,
but it has to wait for a release carrying this change.

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[yingzhehu-TT]

@googlebot I signed it!

Needs further review