fix: merge overlapping date ranges in schedule builder

[KartikLabhshetwar]

What does this PR do?

• Fixes An overlapping working hour results in "could not book the meeting" #22808
• Fixes CAL-6173

Visual Demo (For contributors especially)

Video Demo (if applicable):

Screen.Recording.2026-01-10.at.4.06.45.PM.mov

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 in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

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Summary by cubic

Merges overlapping and adjacent working hours into single continuous ranges in the schedule builder. Fixes booking failures caused by overlapping slots (CAL-6173, #22808).

• Bug Fixes
  • Added mergeDateRanges and applied it to both availability and OOO excluded ranges in buildDateRanges.
  • Merges intervals that overlap or touch after sorting by start time.
  • Added tests for overlapping and adjacent working hours.

^{Written for commit 28cd118. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

[KartikLabhshetwar]

hi @emrysal can i get a review for this?

[cubic-dev-ai]

No issues found across 2 files