fix(core): remove tenantId substring check on file path

[MTarek165]

closes #13745

Description

• splitting file path assumes that it starts with tenantId not namepace and file path reaching there not containing tenantId but since tenantId parameter is not null so it always removes the namespace instead of tenantId causing StringIndexOutOfBounds exception when trying to remove flowId

Solution

• From my investigation I see that always the correct storage uri formula is kestra:///{namespace}/{flowId}/......
  so I removed redundant tenantId splitting

Testing

• Since I can't assert on namespace using the public method since it returns the file content, it is mandatory to isolate it and make it package-private for testing purposes