test(vindex): make PK-vector concurrency tests deterministic

[QuakeWang]

Purpose

PK-vector concurrency tests inferred task overlap from short sleeps and assumed the shared executor had enough physical workers. On constrained or busy CI runners, correct concurrent code could execute serially and fail nondeterministically.

Brief change log

• Ensure the executor capacity required by parallel test cases.
• Replace sleep-based overlap checks with bounded scorer gates.
• Serialize gated tests and verify shared-budget permits and execution waves explicitly.

Tests

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --workspace --features fulltext,vortex -- -D warnings
• cargo +1.91.0 check --locked --workspace --all-targets --all-features
• cargo test --locked -p paimon --all-targets --features fulltext,vortex

API and Format

Documentation