Bump PHPStan to level: max

[turegjorup]

Summary

Final one-line follow-up of the A → B → C series. The 5.0 PRs cleared every max-level PHPStan error in turn; this PR flips phpstan.neon from level: 8 to level: max so the strictest analysis level is locked in for future contributions.

- level: 8
+ level: max

How we got here

• PR PR A: Type constructor $options / $collaborators array shapes #43 typed the constructor $options / $collaborators array shapes (4 errors).
• PR Replace (string) casts on JSON payload values with is_string guards #42 + PR PR B: Document and tighten the exception system; narrow JSON payload accesses #44 narrowed JSON payload accesses and introduced MetadataException / JwksException for malformed IdP payloads (~9 errors).
• PR PR C: Narrow remaining claim accesses and test types for PHPStan max #45 declared getJwtVerificationKeys as array<string, Key>, annotated validateIdToken's $claims with a \stdClass & object{aud, iss, nonce} shape, and added Mockery overload @var annotations on every test mock (the remaining ~7 errors).

Effect going forward

Any future PR that reintroduces a mixed flow, an untyped array key access, a missing @throws, or any of the other strictness-rule violations now fails CI before review. The codebase is locked at PHPStan's highest level.

Test plan

• task analyze:php at level: max — no errors.
• task test:coverage — 100% on OpenIdConfigurationProvider (24/24 methods, 161/161 lines).
• task lint:php — clean.
• task lint:markdown — clean.
• CI matrix on PR.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9f370c6) to head (ebfc625).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##             develop       #47   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        71        71           
===========================================
  Files              1         1           
  Lines            185       185           
===========================================
  Hits             185       185           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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