chore(storage): update storage path validation rule

[phantumcode]

Issue #

Description

• Update StoragePath validation rule to not allow leading / in the path
• Update related unit tests based on the new validation rule change

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.16%. Comparing base (8364412) to head (83861b3).

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Additional details and impacted files

@@                   Coverage Diff                    @@
##           feature/gen2_storage    #3579      +/-   ##
========================================================
- Coverage                 68.32%   68.16%   -0.16%     
========================================================
  Files                      1058     1073      +15     
  Lines                     35656    36256     +600     
========================================================
+ Hits                      24362    24714     +352     
- Misses                    11294    11542     +248     

Flag	Coverage Δ
API_plugin_unit_test	67.50% <ø> (ø)
AWSPluginsCore	66.90% <ø> (ø)
Amplify	47.33% <ø> (ø)
Analytics_plugin_unit_test	81.16% <ø> (ø)
Auth_plugin_unit_test	79.02% <ø> (+0.02%)	⬆️
CoreMLPredictions_plugin_unit_test	59.44% <ø> (ø)
DataStore_plugin_unit_test	82.37% <ø> (ø)
Geo_plugin_unit_test	?
Logging_plugin_unit_test	62.87% <ø> (?)
Predictions_plugin_unit_test	37.16% <ø> (ø)
PushNotifications_plugin_unit_test	87.13% <ø> (ø)
Storage_plugin_unit_test	76.44% <100.00%> (ø)
unit_tests	68.16% <100.00%> (-0.16%)	⬇️

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