Fix #6232 parse list external fields in Apollo Federation

[michaelstaib]

• add a focused regression test that exercises ExternalSetter with an external list field representation
• extend federation ArgumentParser to handle ListValue nodes for external field parsing
• keep existing object/scalar parsing behavior while enabling list conversion to target CLR list types

Fixes #6232

[github-actions]

Fusion Gateway Performance Results

Simple Composite Query

	Req/s	Err%
Constant 2 (50 VUs)	3332.19	0.00%
Ramping 2 (0-500-0 VUs)	3774.18	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	1.13ms	12.20ms	14.74ms	23.86ms	33.13ms	251.04ms
Ramping 2	1.04ms	52.19ms	57.59ms	101.81ms	146.67ms	314.27ms

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Deep Recursion Query

	Req/s	Err%
Constant 2 (50 VUs)	751.80	0.00%
Ramping 2 (0-500-0 VUs)	893.43	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	9.27ms	57.89ms	64.15ms	82.70ms	102.57ms	772.52ms
Ramping 2	3.06ms	208.59ms	230.96ms	479.96ms	542.49ms	1026.23ms

---

Variable Batching Throughput

	Req/s	Err%
Constant 2 (50 VUs)	5687.97	0.00%
Ramping 2 (0-500-0 VUs)	5151.86	0.00%

Response Times

	Min	Med	Avg	P90	P95	Max
Constant 2	0.15ms	8.21ms	8.69ms	14.51ms	17.63ms	51.84ms
Ramping 2	0.20ms	40.12ms	44.55ms	83.09ms	108.51ms	219.20ms

---

Runner 2 = benchmarking-2

Run 22451293511 • Commit 2c053f7 • Thu, 26 Feb 2026 16:38:22 GMT