Remove dead code and fix exception messages in legacy rules

[mglaman]

Part 4 of 9 in the legacy-code audit stack (on top of #1034). Dead code and exception-message cleanups surfaced by auditing the oldest code (2018–2019). No user-facing behavior changes and no release note needed.

What changed

• Extension::parseInfo(): add the missing closing quote in the "Cannot read" exception message.
• EntityFieldsViaMagicReflectionExtension: throw PHPStan's ShouldNotHappenException instead of a bare LogicException so the failure surfaces as an internal error with context.
• PluginManagerSetsCacheBackendRule: return no errors instead of throwing when outside a class, and delete the $cacheKey block. That block computed the constant strings of the cache key argument and then did nothing with them; $hasCacheBackendSet was already true before its continue, so it was unreachable in effect.
• DiscouragedFunctionsRule: drop 'eval'. It is a language construct, not a function, so it parses as Expr\Eval_ and never reached this rule's FuncCall check.

Out of scope

• Whether PluginManagerSetsCacheBackendRule should require a literal cache key. The dead block looks like an abandoned attempt at that. This PR only removes the unreachable code and does not decide it.
• Flagging eval at all. That needs a rule on Expr\Eval_, tracked separately if wanted.

Testing

Full suite, self-analysis, and phpcs are green.

🤖 Generated with Claude Code