Enable exclusions with a single loop guarantee

[pyrech]

Re-enables the exclusions feature from #314 (disabled in #318 because the algorithm could produce several loops).

Rudolph now builds a single chain (Hamiltonian cycle) with randomized backtracking instead of a giver/receiver matching, so associations always form exactly one loop. A step limit bounds the worst case on pathological exclusions.

Also:

• reshuffle on the validate step now catches RudolphException instead of returning a 500
• randomized test with a hidden guaranteed loop to lock the single loop property

Fix #144

[Copilot]

Pull request overview

Re-enables the exclusions feature by changing Rudolph’s assignment algorithm to always produce exactly one loop (a single Hamiltonian cycle), and updates UI/validation behavior accordingly.

Changes:

• Reworked Rudolph::associateUsers() to build a single-cycle chain with randomized backtracking plus a per-attempt step cap.
• Updated the validate “reshuffle” behavior to catch RudolphException and surface the error instead of failing with a 500.
• Expanded test coverage to assert exclusions are respected and that generated associations form a single loop (including randomized stress coverage).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Santa/Rudolph.php	Implements single-loop (Hamiltonian cycle) generation with bounded backtracking and converts chains to associations.
src/Controller/SantaController.php	Enables exclusions for ≤100 participants and handles reshuffle failures by collecting/displaying RudolphException messages.
tests/Santa/RudolphTest.php	Adds assertions for exclusions + single-loop property and introduces randomized exclusions test plus a targeted “forced single loop” case.
templates/santa/exclusions.html.twig	Updates the “not available” message (now explicitly “too many participants”).
templates/content/faq.html.twig	Removes “work in progress” messaging for exclusions in the FAQ entry.

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[damienalexandre]

LGTM