fix: support optional custom formats

[banteg]

Summary

• use the non-optional struct/union type when calling msgpackFormat()
• use the non-optional struct type for custom field-key lookups while unpacking
• add optional struct/union regression tests for custom formats

Bug

The pack/unpack paths already strip optional wrappers into Type = NonOptional(T) when checking for custom methods, but some call sites still invoked those methods on T.

For optional values such as ?MyStruct or ?MyUnion, that meant the code detected MyStruct.msgpackFormat() or MyUnion.msgpackFormat(), then tried to call ?MyStruct.msgpackFormat() or ?MyUnion.msgpackFormat(). Those optional types do not have the custom methods, so optional custom formats failed at compile time.

The same issue affected custom struct field keys during optional struct unpacking: the field enum and msgpackFieldKey() lookup used T instead of the non-optional struct type.

The new tests demonstrate the compile failures first for optional custom struct and union formats.

Tests

• zig build test --summary all failed before the fix with optional custom format compile errors
• ./check.sh --ci

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Reviewing files that changed from the base of the PR and between b4460b5 and 907f0cc.

📒 Files selected for processing (2)

• src/struct.zig
• src/union.zig

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✨ Finishing Touches

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[lalinsky]

Nice, I'm glad someone else found use for these formatting methods.