Add API version 2.15 with conversation_rating behavior documentation (#358)

* Add API version 2.15 with conversation_rating behavior documentation

Adds the API spec for version 2.15 and documents the behavior change
where conversation_rating now returns the latest completed rating
(one with an actual value) instead of just the latest by ID.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add conversation_rating behavior documentation to Unstable spec

Apply the same versioning documentation to the Unstable API spec.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Remove conversation_rating versioning note

Revert description to standard format matching other API versions,
per PR #358 feedback.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Remove Unstable-only features from 2.15 spec

Removes features that should only be in Unstable:
- /macros and /macros/{macro_id} endpoints
- /conversations/{conversation_id}/handling_events endpoint
- /messages/status endpoint
- macro, macro_list, handling_event, handling_event_list schemas
- Macros tag

These features were inadvertently included when 2.15 was derived from
Unstable. Features already in 2.14 (sms_message, whatsapp) are retained.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Rebuild 2.15 spec from 2.14 base with approved Unstable changes

- Start with 2.14 as base, add only approved 2.15 changes from Unstable
- Add new endpoints: Brands, Emails, Fin Voice, Workflow Export, Company Notes
- Add schemas: brand, brand_list, email_setting, email_list, ai_call_response,
  register_fin_voice_call_request, workflow_export, company_note
- Update resolution_state enum: escalated, negative_feedback, procedure_handoff
- Add message_id to email_message_metadata
- Add company field to conversation schema
- Add previous field to attribute updated events
- Add conversation_attribute_updated_by_user schema

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: markdennis22