TW-4619: add "specific_time_availability" to calendars support [WIP, BLOCKED]

[googsvg]

Adds the specific_time_availability field to AvailabilityParticipant, allowing users to override open_hours for specific dates and time ranges when calling POST /v3/calendars/availability. Fully backward compatible — the new field is optional and defaults to null.

License

I confirm that this contribution is made under the terms of the MIT license and that I have the authority necessary to make this contribution on behalf of its copyright owner.

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Note

Low Risk
Adds an optional request-model field and corresponding serialization/tests; no auth, persistence, or behavior changes beyond sending additional JSON when provided.

Overview
Adds support for per-date availability overrides when calling POST /v3/calendars/availability by introducing specific_time_availability on AvailabilityParticipant and a new SpecificTimeAvailability model (date, start, end).

Updates builders and expands serialization/request tests to cover JSON round-trips, backward compatibility when the field is omitted, and request payload generation including the new field; also updates the changelog (plus a local Claude settings tweak).

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.93%. Comparing base (336654d) to head (362f37e).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #304   +/-   ##
=========================================
  Coverage     82.93%   82.93%           
  Complexity      348      348           
=========================================
  Files            36       36           
  Lines          1002     1002           
  Branches         94       94           
=========================================
  Hits            831      831           
  Misses          115      115           
  Partials         56       56           

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