refactor(nestjs-signed-url): restructure signing boundaries

[TorinAsakura]

Task

• Close [Feature] Restructure signed URL package boundaries #397

How to verify

Main scenario

1. Context: @atls/nestjs-signed-url GCS module wiring
   Action: Use SignedUrlModule.gcs({ useValue: storage }) and resolve SignedUrlSigner.
   Expected result: The signer delegates to GcsSignedUrlGateway; the signed-url package does not construct the GCS SDK client.

Additional scenario

1. Context: @atls/nestjs-signed-url GCS client DI
   Action: Use SignedUrlModule.gcsAsync(...) with an imported factory provider and resolve SignedUrlSigner.
   Expected result: The signer uses the injected factory result as the GCS client.

2. Context: @atls/nestjs-signed-url GCS signing options
   Action: Generate a GCS read URL with provider-specific gcs options such as virtualHostedStyle, cname, and queryParams.
   Expected result: The GCS gateway forwards those options into GetSignedUrlConfig without adding provider bags to common signed-url options.

Proofs

• yarn check

{"status":"passed","typecheckFiles":465,"lintFiles":463}

• yarn typecheck packages/nestjs-signed-url

{"status":"passed","files":22}

• yarn test unit --test-reporter=tap from packages/nestjs-signed-url

{"status":"passed","tests":10,"fail":0}

• yarn run build from packages/nestjs-signed-url

{"status":"passed","files":12}

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[TorinAsakura]

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