feat(c): build a catalog-free Table from a resolved schema JSON

[JunRuiLee]

Purpose

Add a catalog-free way to build a read-only Table from an already-resolved Paimon TableSchema, so a non-Java engine (e.g. Doris FE/BE via the C FFI) can rebuild a table for scan/read without a catalog lookup.

Today the only way to obtain a Table is Catalog::get_table, which re-resolves the table — risking schema/snapshot drift versus what the planner (the FE) already resolved — and ties the reader to a filesystem-catalog layout.

This is intentionally minimal and does not introduce a new cross-language DTO. An earlier iteration proposed a dedicated TableDescriptor abstraction on the Java side to bundle path/schema/branch, but the community felt it lacked sufficient motivation: it would re-define Paimon table metadata on the Java side and couple the paimon-rust interface to Doris integration details. That approach is dropped in favor of a generic, catalog-free entry point.

Design

The caller (Doris FE) passes everything a reader needs directly:

• table_path — the table location
• table_schema_json — the full TableSchema JSON, already resolved by the FE (parsed with the existing serde TableSchema shape; not re-loaded or overridden)
• database / table_name — used to synthesize the table Identifier
• branch — selects the branch-scoped managers (schema/snapshot/tag read paths); nullable, defaults to main
• storage_options — used only to build FileIO; they are not merged into the supplied schema

The Rust side constructs the Table directly: no Catalog, no warehouse derivation, and no re-resolution of the FE-provided schema. branch only decides which branch's schema/snapshot/tag paths reads follow; reads can still use FE-generated splits, and Rust-side time travel remains available through the existing read-builder options.

Brief change log

• crates/paimon/src/table/mod.rs: Table::from_resolved_schema(file_io, identifier, location, schema, branch) — preserves the supplied schema as-is, validates the branch name, wires a branch-scoped SchemaManager, sets rest_env = None and no time travel.
• bindings/c/src/table.rs: C FFI paimon_table_from_schema_json(table_path, table_schema_json, database, table_name, branch, storage_options, storage_options_len), mirroring the existing paimon_catalog_get_table handle/free contract, with an ABI signature guard. branch may be null (defaults to main); a non-null pointer is still fully UTF-8 and branch-name validated. Freed via paimon_table_free.
• Historical per-file schemas are still read from the immutable on-disk schema/schema-N by SchemaManager; the resolved schema pins the planned/current schema only.

Tests

• paimon-c FFI tests: build a table from a resolved schema JSON; null-branch defaults to main; invalid branch name (e.g. ../dev) rejected; malformed JSON rejected; null-pointer / null storage_options with non-zero length rejected.

API and Format

• New public API: paimon::table::Table::from_resolved_schema, and C FFI paimon_table_from_schema_json (additive; existing symbols unchanged).
• No new on-disk or wire format, and no new cross-language DTO. Consumes the existing serde TableSchema JSON.

Documentation

None yet (new, evolving C FFI surface for the Doris integration); docs can follow once the integration stabilizes.