feat: support v4 signed url endpoint

[7hokerz]

Description

• Issue: [Emulator] Support generateSignedPostPolicyV4 in Storage Emulator #10488

I added the endpoint and also added test code for the endpoint.

Additional Context

Actually, the latest version of the package has been released, but the existing method,
npm i @google-cloud/storage@latest, failed to download the package.
@google-cloud/storage v7.20.0 release

[gemini-code-assist]

Code Review

This pull request implements support for signed post policy uploads in the GCS emulator by adding a new POST endpoint and a custom multipart/form-data parser. The feedback identifies a bug where the parser could truncate data if a trailing line separator is missing and suggests adding a verification check. Additionally, the reviewer recommends against trimming custom metadata values to maintain parity with GCS behavior and suggests making the boundary extraction logic more robust to handle extra Content-Type parameters.

[7hokerz]

@aalej @cabljac , Could you please take a look at this when you have a moment?

[aalej]

Thanks for submitting this PR @7hokerz! I don't have a good understanding of the Storage emulator, but let me try checking with our engineering team to see if anyone more familiar with the Storage emulator would be able to take a look.

Just to set expectations, we're going on a code freeze, so it might take some time for this PR to get merged.

[7hokerz]

Thanks for submitting this PR @7hokerz! I don't have a good understanding of the Storage emulator, but let me try checking with our engineering team to see if anyone more familiar with the Storage emulator would be able to take a look.

Just to set expectations, we're going on a code freeze, so it might take some time for this PR to get merged.

@aalej , Thank you for your response. I will gladly wait until a storage emulator expert is available. I appreciate your assistance!

[annajowang]

(i am also not a "storage emulator expert". I don't think there currently is one. but if bugs come up after this is merged they can always be fixed)

Left some comments, please address them and the gemini ones as well.
Thanks for your contribution!

[7hokerz]

(i am also not a "storage emulator expert". I don't think there currently is one. but if bugs come up after this is merged they can always be fixed)

Left some comments, please address them and the gemini ones as well. Thanks for your contribution!

@annajowang , Thank you for your feedback! I will take the time to review it and make improvements soon.

[7hokerz]

Note that @google-cloud/storage v7.21.0 is officially out, resolving the path-related issues.

[wiz-9635d3485b]

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