Enable Mosaic reads by default

[JingsongLi]

Summary

Make Mosaic reads always available instead of hiding them behind a Cargo feature, and upgrade paimon-mosaic-core to 0.2.0 so array and map columns are supported by the reader.

Changes

• Treat paimon-mosaic-core as a regular dependency and remove the Mosaic Cargo feature declarations.
• Remove Mosaic feature gates from the core reader registration, unit tests, and DataFusion fixture test.
• Delegate Mosaic Arrow type validation to paimon-mosaic-core and add an array/map round-trip test.
• Remove mosaic from CI feature lists and update docs to describe Mosaic reads as always available.

Testing

• cargo check -p paimon
• cargo check -p paimon --no-default-features --features storage-memory
• cargo check -p paimon-datafusion --no-default-features
• cargo test -p paimon mosaic --lib
• cargo test -p paimon-datafusion --no-default-features --test mosaic_tables

Notes

cargo check -p paimon --no-default-features still requires at least one storage feature because of existing storage enum behavior, so this PR verifies the no-default path with storage-memory enabled.

[leaves12138]

One compatibility concern: removing the mosaic feature name itself is a breaking change for downstream crates that already depend on it.

After this PR, Mosaic is always enabled, but existing dependency declarations such as:

paimon = { version = "0.3.0", features = ["mosaic"] }
paimon-datafusion = { version = "0.3.0", features = ["mosaic"] }

will fail Cargo resolution because neither crate defines a mosaic feature anymore. I verified this locally with a small downstream cargo check, which fails with:

package `check-mosaic-feature` depends on `paimon` with feature `mosaic` but `paimon` does not have that feature

Could we keep a no-op compatibility feature, e.g. mosaic = [] in paimon and mosaic = ["paimon/mosaic"] or mosaic = [] in paimon-datafusion, while still making the dependency non-optional and enabling reads by default? That preserves existing user Cargo.toml files and CI commands while achieving the goal of this PR.

[leaves12138]

Looks good to me. The Mosaic feature compatibility concern is intentionally not a blocker here.