fix: append bucket name to V4 policy URL in emulator mode

[7hokerz]

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Description

This PR fixes a bug where generateSignedPostPolicyV4 returns an incorrect URL (missing the bucket name) when the STORAGE_EMULATOR_HOST environment variable is set.
This change is scoped strictly to this method and does not modify the global apiEndpoint or Service configuration.

Impact

Updated generateSignedPostPolicyV4 in src/file.ts to explicitly append the bucket name to the URL if the client is running in emulator mode and the bucket path is missing.

Testing

Added a new test case in test/file.ts to verify that the bucket name is correctly appended to the URL when STORAGE_EMULATOR_HOST is active.
Ran existing tests: Verified that all existing tests pass (npm test) to ensure no regressions.
No breaking changes are introduced.

Additional Information

This PR is somewhat related https://github.com/googleapis/nodejs-storage/issues/2092.
But This PR avoids those issues by applying a localized fix solely for the V4 Post Policy generation instead of modifying the global service configuration.

Checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated
• Appropriate comments were added, particularly in complex areas or places that require background
• No new warnings or issues will be generated from this change

Fixes #issue_number_goes_here 🦕

issue: #2715