Actually wire the pluggable expression convertor

[AdamGS]

Summary

• Closes Custom ExpressionConvertor is not propagated from VortexSource to VortexOpener #7731

Turns out I didn't wire the expression convertor extension point correctly. This PR both fixes that AND adds a test to make sure this behavior is maintained.

[AdamGS]

introduce new private function because currently FileSource can't be downcasted, so we want to return a real type in tests.

[codspeed-hq]

Merging this PR will degrade performance by 10.6%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
❌ 1 regressed benchmark
✅ 1193 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	runend[10M_i32_runlen_10]	195.1 µs	160.4 µs	+21.63%
⚡	WallTime	runend[10M_i32_runlen_1000]	135.2 µs	97.2 µs	+39.13%
⚡	WallTime	dynamic_dispatch_u32[10M]	161.8 µs	108.5 µs	+49.06%
⚡	WallTime	dynamic_dispatch_u32[10M]	169 µs	115.7 µs	+46.02%
❌	Simulation	bitwise_not_vortex_buffer_mut[128]	246.1 ns	275.3 ns	-10.6%

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_{Comparing adamg/pluggable-expr-convertor-fix (fd87612) with develop (c73dbb2)}