Fix #6203 external required fields in federation entity resolution

[michaelstaib]

• reproduce HotChocolate.ApolloFederation Required external fields returns null #6203 with an _entities query over an extended User entity using [External] + [Requires("firstName")]
• apply ExternalSetter after object entity reference resolution in EntitiesResolver
• add regression test to ensure required external fields are populated before resolving dependent fields

Fixes #6203

[github-actions]

Fusion Gateway Performance Results

Simple Composite Query

	Req/s	Err%
Constant 2 (50 VUs)	3332.77	0.00%
Ramping 2 (0-500-0 VUs)	3833.57	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	1.10ms	12.05ms	14.75ms	24.17ms	33.53ms	251.49ms
Ramping 2	1.17ms	51.55ms	57.18ms	100.06ms	143.10ms	292.88ms

---

Deep Recursion Query

	Req/s	Err%
Constant 2 (50 VUs)	790.80	0.00%
Ramping 2 (0-500-0 VUs)	930.37	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	9.16ms	55.04ms	61.03ms	78.96ms	95.64ms	727.02ms
Ramping 2	3.07ms	199.11ms	221.81ms	458.64ms	516.51ms	841.11ms

---

Variable Batching Throughput

	Req/s	Err%
Constant 2 (50 VUs)	5870.59	0.00%
Ramping 2 (0-500-0 VUs)	5121.55	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	0.15ms	7.94ms	8.42ms	14.01ms	17.23ms	60.57ms
Ramping 2	0.18ms	40.63ms	44.87ms	83.79ms	108.16ms	231.84ms

---

Runner 2 = benchmarking-2

Run 22467054166 • Commit 2015ece • Fri, 27 Feb 2026 00:34:18 GMT

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (cc4976d) to head (bf774e1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff      @@
##   main   #9234   +/-   ##
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