Add on-demand storage fakes and fix root signed URLs

[binaryfire]

Laravel Updates

• #61378 — Allow Storage::fake('ondemand') and Storage::persistentFake('ondemand') to replace anonymous Storage::build() calls. Builds with an explicit logical name retain their own configuration. Forgetting or purging the fake restores normal construction.
• #61093 — Use direct identity and integer assertions in the remaining HTTP, database, process and string tests, preserving their existing behavior.

Additional Hypervel Fixes

• Fix relative signed URLs for the root route. Generation uses /, but validation constructed // and rejected the generated signature. Extend the existing relative-signature test to cover the root route and invalid signatures.
• Reserve ondemand as a configured disk name so a real cached disk cannot replace unrelated anonymous builds. Reject the configuration with a rename instruction, document the difference from Laravel, and preserve explicit logical names and pool identities.

Summary by CodeRabbit

• New Features
  • On-demand storage fakes now support the reserved ondemand disk name and can be reused for matching anonymous disk builds. Explicitly named builds retain their own configuration.
• Bug Fixes
  • Relative signed URLs are now validated using the request path, including checks that reject modified signatures.
• Documentation
  • Filesystem guidance now explains that ondemand is reserved and that configured disks cannot use this name.