Reserve UserAgentFeatureFlags bit 6 (Http2PingHealth = 64) cross-SDK with Java

[jeet1995]

Summary

The Java SDK is adding an HTTP/2 PING keepalive (azure-sdk-for-java#49095) and is reserving bit 6 (value 64) in UserAgentFeatureFlags as Http2PingHealth for user-agent telemetry.

Filing this issue so the .NET SDK reserves the same bit before any future flag claims it, keeping the cross-SDK bitmap aligned per the convention noted in UserAgentFeatureFlags.cs.

Current state

Bit	Value	.NET (UserAgentFeatureFlags.cs)	Java (UserAgentFeatureFlags.java)
0	1	PerPartitionAutomaticFailover	PerPartitionAutomaticFailover
1	2	PerPartitionCircuitBreaker	PerPartitionCircuitBreaker
2	4	ThinClient	ThinClient
3	8	BinaryEncoding	(reserved comment only)
4	16	Http2	Http2
5	32	(free)	RegionScopedSessionCapturing
6	64	(free — request to reserve)	Http2PingHealth (this PR)

Ask

Reserve bit 6 = Http2PingHealth = 64 in Microsoft.Azure.Cosmos/src/Diagnostics/UserAgentFeatureFlags.cs, even as a commented-out placeholder, so the bit is not accidentally reused.

Reserving it does not require .NET to implement the HTTP/2 PING feature — only that the bit is claimed so the user-agent |F<hex> decoder remains comparable across SDKs.

Context

• Java PR: Add HTTP/2 PING for broken connection detection. azure-sdk-for-java#49095
• Java enum (post-merge):
  https://github.com/Azure/azure-sdk-for-java/blob/squad/http2-ping-keepalive/sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/UserAgentFeatureFlags.java
• Feature: scoped HTTP/2 PING keepalive on thin-client gateway channels, modeled on the Rust hyper PING behavior. Surfaces in the user-agent so operators can confirm a given client is running with PING health checks.

A separate cross-SDK reconciliation for bits 3 and 5 (where .NET and Java have already drifted) is out of scope here; raising that as its own discussion if useful.

cc @kushagraThapar