feat: add pluggable transport interface for sagemaker support

[GregHolmes]

Proposed changes

Adds a pluggable streaming-transport interface so the JS SDK can route listen.v1, listen.v2, speak.v1, and agent.v1 connections through a custom transport instead of the default WebSocket. Also adds a reconnect parity flag for clean handoff of retry responsibility to custom transports.

This is the JS half of the cross-SDK pluggable-transport work that's also landed in Python and Java for SageMaker support.

What's new

1. DeepgramTransport / DeepgramTransportFactory interface (src/transport.ts)

   • Hand-maintained type surface that custom transports implement
   • Same factory(url, headers, request) shape as the Python/Java equivalents, with JS-specific request metadata as a third argument

2. transportFactory option on DeepgramClient (wired in src/CustomClient.ts)

   • When set, listen.v1 / listen.v2 / speak.v1 / agent.v1 createConnection paths build a TransportWebSocketAdapter that exposes the SDK's expected ReconnectingWebSocket surface but delegates to the custom transport underneath
   • REST behaviour stays unchanged; only the streaming connection layer is pluggable

3. reconnect flag with auto-disable (commit b82d30b)

   • New reconnect?: boolean option on DeepgramClient
   • Default is true (SDK manages reconnects, current behaviour)
   • When transportFactory is set, auto-disabled to false — the custom transport owns its retry/reconnect lifecycle; double-stacking the SDK's wrapper retries on top would cause storm-on-storm under burst load
   • Callers can still opt back in with explicit reconnect: true
   • Exposed read-only as client.reconnect

Context

• This is the SDK seam that @deepgram/sagemaker (deepgram-js-sdk-transport-sagemaker) plugs into. That package's createSageMakerTransportFactory returns a DeepgramTransportFactory matching this interface.
• The reconnect flag mirrors the same parity work in:
  • deepgram-java-sdk
  • deepgram-python-sdk #720
• The JS half of this work has been validated end-to-end against three live SageMaker endpoints (Nova-3 STT, Flux, Aura TTS), including a 400-concurrent-connection burst against Nova-3 with mean WER 0.10% — matching Java's reference test.

Testing

• pnpm build
• pnpm vitest run tests/unit/transport-factory.test.ts — adapter-path unit coverage including reconnect-flag defaults, explicit reconnect: false, transportFactory auto-disable, and explicit reconnect: true override
• pnpm vitest run tests/unit/custom-client.test.ts
• End-to-end validation against live SageMaker endpoints (Nova-3 STT, Flux, Aura TTS); see the @deepgram/sagemaker package for the transport implementation those tests exercise

Types of changes

• New feature (non-breaking change which adds functionality)

Release impact

• feat: per commit message → minor version bump per release-please
• Fully backwards-compatible — both transportFactory and reconnect are optional kwargs with sensible defaults; existing callers need no code changes
• Hand-maintained files added to .fernignore so Fern regen doesn't overwrite them

Companion packages

This PR provides the SDK-side interface. The first consumer is:

• @deepgram/sagemaker (repo: deepgram-js-sdk-transport-sagemaker) — SageMaker bidirectional HTTP/2 transport, validated end-to-end at 400 concurrent connections

Checklist

• 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)