Throw when middleware class is missing or cannot be resolved

[n0nag0n]

Summary

• Validates string middleware classes at request time instead of silently skipping them
• Throws an exception when a middleware class cannot be autoloaded, matching the error message used for missing route handler classes
• Throws when a middleware class with a container cannot be resolved
• Preserves existing behavior where middleware that only implements before or after is skipped for the other phase

Test plan

• Added testMiddlewareMissingClassThrowsException for route-level middleware
• Added testMiddlewareMissingClassInGroupThrowsException for group middleware
• All 471 existing tests pass

Fixes #696