[TASK-250] KV Tables in CPP

[fresh-borzoni]

Summary

closes #250

KV Tables Support for C++ Bindings

Added UpsertWriter and Lookuper classes to the C++ bindings, enabling full KV table operations on primary-key tables.

UpsertWriter

• Upsert and delete rows by primary key
• Partial updates by column names (NewUpsertWriter(writer, {"user_id", "balance"}))
• Partial updates by column indices (NewUpsertWriter(writer, {0, 1}))

Lookuper

• Point lookups by primary key
• Returns full row with all column values
• Pads PK row to full schema width with Nulls (matching Python behavior)

Note: currently copies over FFI, will be optimized as part of broader #87

[fresh-borzoni]

@luoyuxia @zhaohaidao PTAL 🙏

[leekeiabstraction]

TY for the PR! Left some questions

[fresh-borzoni]

@leekeiabstraction TY for the review.
Addressed comments, PTAL 🙏

[Copilot]

Pull request overview

Adds KV-table (primary-key) operations to the C++ bindings by introducing upsert/delete and point-lookup APIs, plus required row/FFI conversions to match existing Rust/Python behavior.

Changes:

• Add UpsertWriter and Lookuper to the C++ API and Rust/CXX bridge (create writer, upsert/delete/flush, lookup).
• Add schema-aware GenericRow creation (Table::NewRow) and name-based setters for more ergonomic row building.
• Improve/extend Rust↔FFI row conversions (including TinyInt/SmallInt and safer error propagation), and add a KV usage example + build targets.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
bindings/cpp/src/types.rs	Improves datum conversions (TinyInt/SmallInt, timestamp error handling), returns Result instead of panicking, adds internal_row_to_ffi_row for lookup results.
bindings/cpp/src/table.cpp	Implements C++ UpsertWriter/Lookuper, adds Table::NewRow() and schema column-map caching.
bindings/cpp/src/lib.rs	Extends the CXX bridge with new KV types/APIs; adds Rust-side UpsertWriter/Lookuper implementations and lookup result wiring.
bindings/cpp/src/ffi_converter.hpp	Minor formatting change.
bindings/cpp/include/fluss.hpp	Adds schema-aware name-based setters on GenericRow, new KV API declarations.
bindings/cpp/examples/kv_example.cpp	New end-to-end KV example covering upsert/delete/lookup and partial updates.
bindings/cpp/examples/admin_example.cpp	Minor formatting change.
bindings/cpp/CMakeLists.txt	Builds the new KV example binary.
bindings/cpp/BUILD.bazel	Adds Bazel target for the KV example binary.

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[fresh-borzoni]

Addressed comments

[luoyuxia]

@fresh-borzoni Thanks. LGTM!