fix: scope API mapping cache entries to their domains

[Shubham-Padkonde]

Issue #: Fixes #2134.

Description of changes:

The REST and WebSocket API mapping models both use api_mapping as their resource name. When their mount paths match, RemoteState reuses an existence result from the other domain because its cache key does not include the domain passed to resource_exists.

After an existing REST mapping is checked, a new WebSocket mapping can therefore appear to exist. Planning then fails while reading deployment values for the new domain with ValueError: Deployed values for resource does not exist: websocket_api_custom_domain. In the opposite order, an existing mapping can incorrectly appear to be missing.

Include the domain name in API-mapping cache keys. Repeated checks within the same domain stay cached, and other resource keys are unchanged. Adds a release note and three regression cases: both existence-result orders, with exact underlying calls and repeated cache hits, plus planning a new WebSocket domain after an existing REST domain using real RemoteState and mocked AWS responses. All three fail on unmodified master; the planner test reproduces the reported exception exactly. All 111 planner tests pass after the fix.

Validation: 1,400 tests passed and 27 skipped on Linux/Python 3.11.15 with pinned development requirements. Full make prcheck passed, including linting, documentation link checking and HTML build, and mypy (63 source files).

No live AWS deployment or other Python/OS matrix was run. Implementation and validation were assisted by OpenAI Codex; no independent human review is claimed.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.