fix(cli): surface YAML and AsyncAPI errors from docs serve

[SarthakB11]

Description

Refs #2709 (diagnostic portion).

Per @Sehat1137's recommendation on #2858 ("a smaller PR limited to that behavior, with deterministic parsing and regression tests"), this is the diagnostic-only redo: no new CLI options, no parser plugin, default yaml.safe_load preserved. The pluggable-parser proposal from #2709 is intentionally deferred per maintainer feedback.

faststream docs serve <asyncapi.yaml> previously collapsed three different failure modes into one opaque SCHEMA_NOT_SUPPORTED message:

• YAML parse errors were not surfaced. yaml.safe_load ran outside any try, so the user either got an uncaught traceback or, more often, a downstream pydantic failure on the garbage that partial parsing produced.
• AsyncAPI validation errors were dropped by contextlib.suppress(ValidationError), so there was no indication of which schema version was tried or why it failed.
• YAML scalar gotchas like protocolVersion: 3.2 (parsed as a float, rejected by AsyncAPI's string type) gave no hint that the value just needed quoting. This is the exact repro from Bug: PyYAML cannot correct parse asyncapi.yaml #2709.

Fix

Edits only _parse_and_serve in faststream/_internal/cli/docs.py:

1. Wrap yaml.safe_load in try / except yaml.YAMLError, echo the parser exception, exit 1.
2. Replace suppress(ValidationError) with an explicit collector that records (version_label, ValidationError). When neither v3.0 nor v2.6 validates, print SCHEMA_NOT_SUPPORTED followed by both per-version error blocks on stderr.
3. For each block, if any sub-error has type == "string_type" and an int | float | bool input, append a one-line hint pointing at the location and suggesting YAML quoting (e.g. '3.2').

Default parser is unchanged. _parse_and_serve's public signature is untouched. dto.py is untouched. No new modules, no parser registry, no --yaml-parser flag.

Tests

Two regression tests in tests/cli/test_asyncapi_docs.py:

• test_serve_asyncapi_yaml_unquoted_scalar_reports_hint mutates the existing yaml_asyncapi_doc fixture to protocolVersion: 3.2, asserts exit code 1 and the v3.0 / v2.6 error enumeration plus the quoting hint.
• test_serve_asyncapi_reports_yaml_parse_error feeds malformed YAML (foo: [unclosed) and asserts the parser exception is surfaced rather than swallowed.

Both follow the existing faststream_cli + generate_template + wait_for_stderr pattern in the same file.

Type of change

• Bug fix (a non-breaking change that resolves an issue)

Checklist

• My code adheres to the style guidelines of this project (just lint shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running just test-coverage
• I have ensured that static analysis tests are passing by running just static-analysis
• I have included code examples to illustrate the modifications