Upgrade Subscriptions.Nats to official NATS v2 client

[michaelstaib]

• replace AlterNats.Hosting with official NATS v2 packages (NATS.Client.Core and NATS.Extensions.Microsoft.DependencyInjection)
• migrate NatsTopic subscription lifecycle to the NATS v2 async subscription API with explicit cancellation/disposal handling
• pass cancellation tokens through NATS publish calls
• update Subscriptions.Nats docs and DI guidance to AddNatsClient(...)
• add an integration test that verifies topic prefix isolation across two servers sharing the same NATS broker

Closes #6809.
Closes #7532.

[github-actions]

Fusion Gateway Performance Results

Simple Composite Query

	Req/s	Err%
Constant 2 (50 VUs)	3158.45	0.00%
Ramping 2 (0-500-0 VUs)	3725.98	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	1.19ms	12.77ms	15.55ms	25.15ms	34.53ms	261.68ms
Ramping 2	1.36ms	53.44ms	58.86ms	101.64ms	148.17ms	290.95ms

---

Deep Recursion Query

	Req/s	Err%
Constant 2 (50 VUs)	745.91	0.00%
Ramping 2 (0-500-0 VUs)	887.82	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	9.57ms	57.57ms	64.70ms	83.37ms	101.32ms	750.86ms
Ramping 2	2.97ms	208.25ms	232.41ms	481.99ms	537.47ms	804.67ms

---

Variable Batching Throughput

	Req/s	Err%
Constant 2 (50 VUs)	5674.70	0.00%
Ramping 2 (0-500-0 VUs)	5269.11	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	0.16ms	8.23ms	8.71ms	14.55ms	17.60ms	64.11ms
Ramping 2	0.19ms	39.16ms	43.59ms	81.61ms	105.81ms	208.04ms

---

Runner 2 = benchmarking-2

Run 22439451158 • Commit 80b382e • Thu, 26 Feb 2026 11:22:07 GMT

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (6a3b7a3) to head (86181eb).
⚠️ Report is 820 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #9206       +/-   ##
==========================================
- Coverage   74.16%       0   -74.17%     
==========================================
  Files        2677       0     -2677     
  Lines      140790       0   -140790     
  Branches    16371       0    -16371     
==========================================
- Hits       104421       0   -104421     
+ Misses      30774       0    -30774     
+ Partials     5595       0     -5595     

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