Add endpoint to reset the per-SP WAYF remember-choice cookie

[kayjoosten]

What

Implements #2085, adding GET /reset-remember-wayf: a public endpoint that
clears the per-SP rememberedidps cookie (introduced in #2065 / #2091) and
redirects the user to an operator-configured URL.

Stacked on #2091

This branches from wayf-remember-choice-core (#2091, not yet merged),
since it depends directly on the RememberedIdpCookie service introduced
there. The diff shown here is the incremental change for #2085 only; once
#2091 merges to main, this PR's base should be retargeted to main.

Details

• New ResetRememberedWayfController reuses the existing
  RememberedIdpCookie service: normalize() computes the valid entry
  count for logging, clear() removes the cookie using the same
  SameSite/secure flags it was written with. Both only run when the
  cookie is actually present — a visitor with no cookie is redirected
  without any log noise.
• Logs 'WAYF-remember-my-choice cookie removed (had %d entries)' at
  INFO level, matching the sibling log message conventions from Remember choice for WAYF per SP #2065.
• The controller validates its configured redirect URL at construction
  time and throws if it's blank, so a missing
  wayf.reset_choice_per_idp_redirect fails fast instead of silently
  redirecting nowhere.
• New config wayf.reset_choice_per_idp_redirect defaults to a working
  dev URL in parameters.yml.dist (matching the convention of other
  wayf.* dev defaults), so the endpoint works out of the box in
  dev/CI while still requiring operators to set a real destination in
  production.
• The functional test asserts the redirect target against the
  configured parameter rather than merely checking for a non-empty
  Location header — this app has a global exception listener that
  also turns uncaught exceptions into a 302, which would otherwise let
  a misconfiguration pass unnoticed. (Verified locally: temporarily
  blanking the config reproduces exactly this failure mode, and the
  strengthened assertion catches it.)

Testing

All run inside the Docker dev container (PHP 8.5):

• phpmd / phpcs / phpcs-legacy / docheader: clean.
• eb4: 244/244. unit: 1003/1003 (999 baseline + 4 new). functional
  (APP_ENV=test): 121/121 (119 baseline + 2 new). integration:
  105/105.
• Behat default suite: 300/300 scenarios, 5557/5557 steps.
• Twig lint: 114/114. yarn lint: clean.

Refs: #2085