Support 500 server sessions: load test, default MaxSessionCount=500, bottleneck analysis

[Copilot]

Description

Adds a 500-session server load test, raises the default session limit to 500, and documents the bottlenecks found while profiling the server at that scale.

• Default config: ServerConfiguration.MaxSessionCount default changed 100 → 500 (ApplicationConfiguration.cs). Updated the default-value round-trip tests accordingly.
• Load test: new [Explicit] ServerManySessionsLoadTestAsync in LoadTest.cs — 500 sessions, each a ManagedSession with one slow-publishing (1000 ms) subscription (added via ManagedSession.AddSubscription on the V2 SubscriptionManager) on a shared value node; a writer drives changes for 60 s; asserts every session receives notifications under load. Session count and duration are fixed defaults (500 sessions, 60 s). Connect is throttled (~ProcessorCount × 4), reuses a single resolved endpoint instead of per-session discovery, and retries transient connect timeouts, and the fixture runs at Warning log level to avoid per-publish logging overhead.
• Test framework: ClientTestFramework gains a settable MaxSessionCount (applied to the server config) so fixtures can host 500+ sessions.
• Docs: Docs/SessionScalability.md (linked from Docs/README.md) covers the new default and the profiling findings.

Bottlenecks found (ordered by impact):

• Connection establishment dominates and is CPU-bound — Basic256Sha256 RSA handshakes serialize on cores; bulk connect throughput scales with core count, and bursts surface as BadRequestTimeout. Mitigated client-side via throttling, single-endpoint reuse, and retries.
• Request-thread-pool starvation during connect storms causes transient Publish timeouts (shared MaxRequestThreadCount pool).
• Debug-level per-request logging measurably amplifies load.

Related Issues

Checklist

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• ] I have signed the [CLA and read the CONTRIBUTING doc.
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✅ marcschier
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[marcschier]

I am not sure this is a good place for this doc, it would be better to add something to benchmarks.md or sessions.md, but not in its verbosity, ideally more "This implementation has been tested to achieve x concurrent sessions (client/server) with n subscription x m items with x config. (maybe some matrix) with the following CPU and memory usage. Then in addition to the base line test here, we create a macro benchmark and produce some numbers. What do you think?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.78%. Comparing base (ba027ae) to head (a488a64).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3922      +/-   ##
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- Coverage   72.79%   72.78%   -0.01%     
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  Files         917      917              
  Lines      149563   149563              
  Branches    25918    25918              
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- Hits       108875   108861      -14     
- Misses      31236    31243       +7     
- Partials     9452     9459       +7     

see 13 files with indirect coverage changes

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