chore(api/auth): fix clippy failure for DLF default signer identifier

[freedom3219]

Purpose

Fix the CI failure from cargo clippy --all-targets --workspace -- -D warnings.

cargo clippy --all-targets --workspace -- -D warnings
    Checking paimon v0.0.0 (/workspaces/paimon-rust/crates/paimon)
error: associated constant `IDENTIFIER` is never used
   --> crates/paimon/src/api/auth/[dlf_signer.rs:160](http://dlf_signer.rs:160/):15
    |
159 | impl DLFDefaultSigner {
    | --------------------- associated constant in this implementation
160 |     pub const IDENTIFIER: &'static str = "default";
    |               ^^^^^^^^^^
    |
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`

error: could not compile `paimon` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `paimon` (lib test) due to 1 previous error

The failure was caused by a duplicated DLF signer identifier definition in the auth factory. DLFDefaultSigner::IDENTIFIER existed but was not referenced by the factory code path, which triggered a dead_code warning and was promoted to an error by Clippy in CI.

Brief change log

• Remove duplicated "default" / "openapi" identifier definitions from DLFAuthProviderFactory
• Reuse DLFDefaultSigner::IDENTIFIER and DLFOpenApiSigner::IDENTIFIER as the single source of truth
• Update auth factory tests to assert against the shared signer identifiers
• Add a test to verify the default signer identifier is returned when URI input is missing

Tests

• cargo fmt --all
• cargo test -p paimon
• cargo clippy --all-targets --workspace -- -D warnings
• cargo test --workspace

API and Format

Documentation

[JingsongLi]

+1