feat: add pluggable transport interface for SageMaker support

[GregHolmes]

Proposed changes

• Adds a hand-maintained DeepgramTransport / DeepgramTransportFactory interface for replacing the default streaming transport in the JS SDK
• Wires transportFactory into CustomDeepgramClient so listen.v1, listen.v2, speak.v1, and agent.v1 can use custom transports without modifying Fern-generated clients
• Exports the new transport types, adds unit coverage for the adapter path, and freezes the new hand-maintained surface in .fernignore

Context

• This follows the same integration boundary used in the Python and Java SDKs for SageMaker support
• REST behavior stays unchanged; only the streaming connection layer becomes pluggable
• The transport factory keeps the Python/Java-style factory(url, headers) shape and adds JS-specific request metadata as a third argument

Testing

• pnpm build
• pnpm vitest run tests/unit/transport-factory.test.ts
• pnpm vitest run tests/unit/custom-client.test.ts

Types of changes

What types of changes does your code introduce to the community JavaScript SDK?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update or tests (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc
• I have lint'ed all of my code using repo standards
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments