feat: Add AsyncAPI support in the stats command

[tibisabau]

What/Why/How?

Added AsyncAPI 2.x and 3.x support to the stats command to provide statistics for AsyncAPI documents, similar to existing OpenAPI support. The stats command now counts channels, operations, schemas, references, tags, parameters, and external docs for AsyncAPI specifications.

Why: Since AsyncAPI support was added for lint and bundle commands, the stats command should support it as well for consistency and completeness.

How: Extended the Stats visitor in packages/core/src/rules/other/stats.ts to handle AsyncAPI-specific node types (ChannelMap, NamedChannels, NamedOperations) and map them to appropriate metrics (channels → pathItems, operations → operations).

Reference

Closes #2353.

Testing

• Added 3 new E2E test cases covering AsyncAPI 2.x and 3.x stats in stylish and JSON formats
• All tests pass successfully with generated snapshots
• Verified stats output for both AsyncAPI versions:
  • AsyncAPI 2.x: 4 channels, 4 operations, 18 references, 17 schemas, 12 external docs, 11 tags
  • AsyncAPI 3.x: 1 channel, 1 operation, 2 references

Screenshots (optional)

N/A

Check yourself

• Code changed? - Tested with Redoc/Realm/Reunite (internal)
• All new/updated code is covered by tests
• New package installed? - Tested in different environments (browser/node)
• Documentation update considered

Security

• The security impact of the change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

🦋 Changeset detected

Latest commit: d1b8913

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@redocly/openapi-core	Major
@redocly/cli	Major
@redocly/respect-core	Major

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[DmitryAnansky]

PathItem in OAS is not quite the same as Channel in AsyncAPI, although they serve analogous purposes in their respective specifications.
Have you considered a different accumulator options? Like maybe have a separate collector and tune the output with a different label, like Channels?

[DmitryAnansky]

I can see 3 refs in your file, but snapshot has only 2 reported references.

      UserSignedUp:
        $ref: '#/components/messages/UserSignedUp'
operations:
  sendUserSignedup:
    action: send
    channel:
      $ref: '#/channels/userSignedup'
    messages:
      - $ref: '#/channels/userSignedup/messages/UserSignedUp'

[DmitryAnansky]

Have you considered a different implementation approach using detectSpec with separate stats handlers for each specification? This would help to cleanly separate the concepts of different specifications.
In case specs related, e.g. async2/async3, some implementation can be reused/extended.