fix: validate storage name and alias values

[vdusek]

Summary

Storage aliases were not validated the same way storage names are. This change:

• Adds validation for storage aliases (previously unvalidated), allowing the same simple identifier characters used for names plus the reserved __default__ alias.
• Ensures a storage name or alias always maps to a single subdirectory inside the configured storage directory, both via the high-level API and when the file-system storage clients are used directly.

Includes unit tests for the new validation and for the file-system clients.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.03%. Comparing base (76927d7) to head (7870a85).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/crawlee/storages/_utils.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1950      +/-   ##
==========================================
+ Coverage   92.98%   93.03%   +0.04%     
==========================================
  Files         167      167              
  Lines       11712    11722      +10     
==========================================
+ Hits        10890    10905      +15     
+ Misses        822      817       -5     

Flag	Coverage Δ
unit	93.03% <95.00%> (+0.04%)	⬆️

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[Pijukatel]

One thing confuses me.

This alias: with/slash

• It is not a valid alias at the high-level API. (rejected by validate_storage_alias)
• It is a valid alias when using the storage client directly. (approved by validate_subdirectory)

Is there a specific reason for it?

[vdusek]

One thing confuses me.

This alias: with/slash

• It is not a valid alias at the high-level API. (rejected by validate_storage_alias)
• It is a valid alias when using the storage client directly. (approved by validate_subdirectory)

Is there a specific reason for it?

Oh, good catch, it should be rejected by both of course, I'll fix it.