feat: Instant Book: Added strategy where busy intervals are retrieved from calendar feed

[tuj]

NB! Builds on #433

Link to issue

#436

Link to ticket

https://leantime.itkdev.dk/#/tickets/showTicket/7208

Description

• Added strategy to "Instant Book" where busy intervals are retrieved from calendar feed.
• Fixed instant booking template error handling.
• Fixed bugs when fetching intervals.
• Fixed calendar test issue following change to 10 min default update interval.

Checklist

• My code is covered by test cases.
• My code passes our test (all our tests).
• My code passes our static analysis suite.
• My code passes our continuous integration process.

[turegjorup]

Local review follow-up

Worked through the review findings on this branch. Five commits pushed on top of the original head (7b4318b3..3b9f1e64).

Fixed

Concern	Resolution	Commit
406 (Not Acceptable) for INSTANT_BOOK_BUSY_INTERVALS_SOURCE misconfig	Constructor guard throws \InvalidArgumentException → 500; default arm dropped as unreachable	ca263e37
406 for slide-config errors (null feed / null source / non-calendar type)	Symfony\…\UnprocessableEntityHttpException → 422 (admin-fixable data state, not Accept-header negotiation)	557fc4fb
Pre-booking Graph call looked like wasted overhead	Comment rewritten: the resources here are not configured to block conflicts at the Graph API level — a conflicting POST would silently succeed — so the pre-check is required for correctness, not an optimisation. The 409 catch is a backstop for resources that do reject conflicts	a7ac945e
CACHE_KEY_BUSY_INTERVALS_PREFIX was a fixed key — different slides could read each other's data for up to 15 min	Extracted to getBusyIntervalsCached(); key now built from sorted resource set + from/to at minute precision	91d028ed
No test coverage for the new conflict pre-check	Extracted to assertSlotFree(); added tests for busy → ConflictException and empty → pass	3b9f1e64

Verified safe (no change needed)

• Frontend apiUrl removal in calendar-single-booking.jsx — verified the change is a bug fix, not a regression. slide["@id"] is an absolute path (/v2/slides/<ulid>) per API Platform's default url_generation_strategy = 1 (ABSOLUTE_PATH); confirmed by existing test assertions in LayoutsTest.php. The previous ${apiUrl}${slide["@id"]}/action produced broken URLs because apiUrl came from localStorage populated with configData.apiEndpoint, which ClientConfigController does not actually return — so apiUrl was the literal string "undefined" in any non-fallback config.

Out of scope / FYI

• KobaFeedType timestamp shape — dropped; the feed type is @deprecated, defensive code not warranted.
• Polling cadence 5s → 60s — UX choice, documented in CHANGELOG.
• phpunit.xml.dist disabled → weak — minor scope creep; could be mentioned in the description or split if you want cleaner history.

Tooling: validated via task coding-standards:php:check, task code-analysis, task test:api -- tests/Interactive/InstantBookTest.php after each change. All clean.