Emulator: add direct connection REST APIs

[Liangying.Wei (vicancy)]

Relationship

This is a focused slice extracted from the umbrella emulator PR #1048. It preserves that PR's emulator semantics, including accepting valid messageTtlSeconds values for immediate delivery without modeling expiration.

Summary

• add authenticated connection presence and direct-send REST operations
• deliver text, JSON, and binary data through raw, JSON, and reliable client paths
• validate API versions, request-scoped access-key tokens, content, metadata, and TTL ranges
• verify compatibility with Azure.Messaging.WebPubSub 1.6.0

Validation

• 116 emulator tests passed in Release
• official .NET server SDK integration test passed
• valid message TTL values are accepted and delivered immediately
• adversarial runtime comparison found no merge blockers
• git diff --check passed

Deliberate gaps

• message expiration is not modeled; valid TTL values are accepted and otherwise ignored
• preview API versions and other REST operations remain unsupported