fix(availability): don't offer round-robin slots when a mandatory fixed host is unavailable

[shafi-VM]

What does this PR do?

Fixes #29800

getAggregatedAvailability dropped the fixed-host constraint whenever the fixed hosts' intersection was empty — but that's empty both when there are no fixed hosts and when fixed hosts exist but can't overlap (a mandatory host is busy). fixedDateRanges.length ? [fixedDateRanges] : [] conflated them, so a ROUND_ROBIN event with a busy fixed host leaked slots driven by the round-robin hosts alone — slots the booking layer then rejects with FixedHostsUnavailableForBooking. Gating on fixedHosts.length instead forces an empty fixed-host intersection to make the aggregate empty.

Visual Demo (For contributors especially)

Backend logic fix with no UI surface — demonstrated with a red→green unit test rather than a recording (see How should this be tested?).

Video Demo (if applicable):

N/A — no UI change.

Image Demo (if applicable):

N/A — no UI change.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox. — N/A (no developer-facing API or docs change)
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

• Data: a ROUND_ROBIN event with a fixed host + round-robin host(s); make the fixed host busy while a round-robin host is free.
• Expected: no slots are offered for the busy fixed host's times (previously they were offered and then failed at booking with FixedHostsUnavailableForBooking).
• Automated: new unit test in getAggregatedAvailability.test.ts asserts a ROUND_ROBIN input with a busy fixed host + an available round-robin host returns [] — verified red before the fix, green after; the other 10 tests in the file and the broader availability/schedules suites (213 tests) are unaffected.

Checklist

• N/A — follows the contributing guide and style guidelines, tests included, PR is small (2 files, ~28 lines).

[github-actions]

Welcome to Cal.diy, @shafi-VM! Thanks for opening this pull request.

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• This is Cal.diy, not Cal.com. Cal.diy is a community-driven, fully open-source fork of Cal.com licensed under MIT. Your changes here will be part of Cal.diy — they will not be deployed to the Cal.com production app.
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[coderabbitai]

📝 Walkthrough

Walkthrough

Updated getAggregatedAvailability to retain the fixed-host constraint when fixed hosts have no overlapping availability, resulting in no aggregated slots. Added a Vitest regression test covering an unavailable mandatory fixed host alongside an available round-robin host.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The code change and new test address #29796 by preserving fixed-host constraints and returning no slots when a fixed host is busy.
Out of Scope Changes check	✅ Passed	The PR only changes the targeted availability logic and adds a regression test, with no unrelated scope detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title clearly summarizes the main availability fix and matches the changed behavior.
Description check	✅ Passed	The description accurately explains the bug, fix, and regression test added in this PR.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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