[core] Unify the REST User-Agent format

[sundapeng]

Purpose

REST catalog requests from paimon-rust carry no User-Agent, since reqwest sets none by default, so these clients can't be attributed in the catalog server's request logs.

REST requests now carry Paimon's unified User-Agent format, <module>(<transport>[;<feature>...])[ <extended>], the one object storage requests adopt as well (#991).

Brief change log

• The REST client sends paimon-rust/<version>(reqwest), built from the catalog options user-agent.module, user-agent.features and user-agent.extended, which object storage requests read too. The version is a compile-time constant.
• A user-set header.User-Agent option still takes precedence and is the only User-Agent sent.
• The DLF ECS token client sends the default.

Examples:

Client and options	User-Agent
REST catalog	paimon-rust/0.4.0(reqwest)
user-agent.features=Flink, user-agent.extended=vvr	paimon-rust/0.4.0(reqwest;Flink) vvr
header.User-Agent=starrocks/user	starrocks/user

Related: #991 (object storage User-Agent), apache/paimon#10308 (Java REST), apache/paimon#10309 (Python REST).

Tests

• api:: unit tests: new cases on a local HTTP endpoint for the default, the new options and the header.User-Agent override; 92 passed, plus rest_api_test (45) and rest_catalog_test (79). Also checked on a live DLF REST catalog: its request log shows the configured features and extended part.
• cargo fmt --all -- --check and cargo clippy --locked --all-targets --workspace --features fulltext,vortex -- -D warnings.

API and Format

No.

Documentation

No.

[JingsongLi]

Requirement fit: SUPPORTED — request attribution in REST catalog logs has end-to-end operational value. Implementation: FINDINGS at 70cf3bf2.

[P2] Refresh the User-Agent after merging /v1/config options (crates/paimon/src/api/rest_api.rs:149-152). The client is constructed from the initial options, while the normal RESTCatalog path subsequently merges server defaults/overrides and refreshes only auth/header.*. Consequently the newly added user-agent.* configuration is ignored when supplied by the catalog server, even though it is present in api.options(). I reproduced this with a local HTTP server returning default user-agent.features=ServerFeature and override user-agent.extended=catalog-tag: the merged options contain both, but the following list-databases request sends paimon-rust/0.4.0(reqwest) instead of paimon-rust/0.4.0(reqwest;ServerFeature) catalog-tag. This defeats the configured attribution for clients configured through the catalog. Please update/rebuild the default User-Agent from the merged options after bootstrap, preserving the explicit header.User-Agent precedence, and add a config_required=true HTTP regression. The current new header tests all use false.

Verification: 92/92 API unit tests, 45/45 REST API integration tests, and 79/79 REST catalog integration tests passed. The temporary config-negotiation regression failed with the header mismatch above; the test edit was restored. Diff check and current-main merge-tree passed; all 14 head CI checks are green.

[sundapeng]

Thanks, good catch. The REST client was built from the initial options only, so user-agent.* delivered through /v1/config was ignored. RESTApi::new now rebuilds the default User-Agent from the merged options after bootstrap, and an explicit header.User-Agent still takes precedence. Added a config_required=true regression where the server returns user-agent.features as a default and user-agent.extended as an override (fixed in 7979517).

[JingsongLi]

Re-reviewed 7979517f after the fix. The previous P2 is resolved: the original config-required HTTP regression now sends the server-provided features/extension, and the explicit header.User-Agent precedence test passes. The rebuilt client preserves the auth function and existing timeout/default-header behavior.

Validation on the updated head: 94 API unit tests, 46 REST API integration tests (including the original reproducer), and 79 REST catalog integration tests passed. Current-main merge check is clean. No further actionable code finding from this follow-up; remaining CI runs should finish before merge.