feat(c): add bounded commit-message range export - #1330

Purpose

Linked issue: Closes #1327.

Related to #910 and #1257.

An embedding engine needs to export bounded ranges before passing commit bodies to a size-limited
RPC. The existing API serializes every message before a caller can check the
resulting size.

Prepared with AI assistance.

Brief change log

• Add CommitMessage::serialize_with_limit(max_bytes), checking before each
  output append while preserving the existing serializer and Java v14 bytes.
• Add paimon_commit_messages_count(messages, count_output) and
  paimon_commit_messages_serialize_range(messages, offset, count, max_bytes).
• Reuse the existing byte-array result and free function. Validate range
  overflow and bounds; return no partial output on failure and preserve the
  source handle for a smaller-range retry.

Tests

Validated with the existing Rust 1.97.0 toolchain on macOS arm64:

• cargo +1.97.0 test --locked -p paimon-c --lib: 95 passed, including the existing
  separate worker/coordinator process test.
• New coverage checks Java fixture equality, exact limits, one-byte-smaller
  limits, cumulative limits, nonzero offsets, empty/invalid/overflowing ranges,
  null handles and output pointers, retry, output lifetime and Format Table
  rejection.
• cargo +1.97.0 test --locked -p paimon --lib table::commit_message::tests: 3 passed.
• cargo +1.97.0 fmt --all -- --check and git diff --check: passed.
• cargo +1.97.0 clippy --locked -p paimon-c --all-targets -- -D warnings: passed.

Not run locally: CI-pinned Rust 1.98.0 checks, the full workspace/Vortex
suite, or the Linux/MSRV matrix. The results above cover the listed commands.

API and Format

Additive Rust and C APIs. Existing full export and Java v14 body bytes remain
unchanged. No Java list wrapper, storage format or engine-specific framing is
added. The limit bounds the sum of body lengths, not peak allocations or RSS:
temporary metadata encoding, vector capacity and ownership conversion remain.

Documentation

Add a range-export/retry example and explain byte limits and ownership in the
C integration guide.