fix: add space in decimal type serialization for cross-engine compat

[SreeramGarlapati]

Summary

iceberg-rust serializes the decimal type as decimal(P,S) (no space), while the Iceberg spec, Java, and Python all use decimal(P, S) (space after comma). This mismatch can cause issues when metadata written by iceberg-rust is read by strict parsers in other implementations.

What changed

• serialize_decimal: decimal({precision},{scale}) → decimal({precision}, {scale})
• Display for PrimitiveType: same fix for the decimal arm
• Updated the one test fixture that asserted the old format

Why this is safe

The deserializer already uses .trim() on both precision and scale components, so it accepts both decimal(9,2) and decimal(9, 2). Existing metadata files with the old format continue to be readable.

Closes #2534

Test plan

• All 1300+ iceberg crate tests pass
• Workspace compiles cleanly
• Backwards compat: deserializer handles both formats via trim()

[mbutrovich]

LGTM, thanks @SreeramGarlapati!

[xanderbailey]

Interestingly the spec makes it seem like this is legitimate both with and without the space? It would appear that if other consumers are failing here, they're not compliant?

Interested to know what other people in the community think the right approach here is? It's true that this fix will align with other clients but I also feel like perhaps we either need to clarify the spec or improve the other clients to be compliant?

[mbutrovich]

Changing my approval after the better investigation by @xanderbailey, thanks for double-checking!

[kevinjqliu]

Interestingly the spec makes it seem like this is legitimate both with and without the space? It would appear that if other consumers are failing here, they're not compliant?

i feel like thats a formatting issue on the spec and not intention to have both the form with and without the space.

That said, the general principle with ser/de for iceberg project is Postel’s Law

Be liberal in what you accept, and conservative in what you send.

This is a great place to clarify the spec 😄 thanks for catching it!