Diagnose vector type aliases used as unit fields

[dedsec-terminal]

We would previously abort with an internal compiler error when a type alias resolving to vector<T> was used as a unit parsing field.

This patch updates the validator to diagnose this as an invalid sequence syntax error instead of aborting, matching the diagnostic produced for direct vector<T> field syntax.

Closes #2445.

[Copilot]

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[codspeed]

Merging this PR will not alter performance

✅ 43 untouched benchmarks

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_{Comparing dedsec-terminal:fix/2445-vector-alias-diagnostic (5da9852) with main (a36b8c4)}

[evantypanski]

Thanks for the fix

is there a reason this keeps the validation with the same error in parser.yy? I would prefer they both go through the same validation, since they're diagnosing the same thing. Might be worth a bit larger change to get that to work if it doesn't currently.

[dedsec-terminal]

Good point, thanks. I pushed 5da9852 to remove the parser-time diagnostic, so direct vector fields and aliases now go through the same validator path. I also refreshed the existing direct-syntax regression to remove obsolete &length parse errors, ensuring it exercises that shared validation.

[evantypanski]

LGTM, thanks :)

Seems like soon this'll be due for a cleanup, it's been a while since that syntax was supported. This fixes the internal error so I'm happy with this as-is.