Add sync-first runtime bridge

[Saurabh Singh (saurabh500)]

Description

Adds a supported mssql_tds::runtime::block_on_sync_first bridge for synchronous consumers of native TDS futures.

The bridge pins exactly one future, polls it once under the supplied Tokio runtime with the standard no-op waker, returns directly when buffered work is ready, and otherwise resumes that same pinned future through Runtime::block_on. Entered Tokio contexts are rejected before polling so a stateful protocol future cannot be partially advanced under an invalid fallback policy.

This preserves strict row-at-a-time and column-at-a-time streaming. It does not batch, prefetch, or materialize whole rows.

Correctness coverage

• Ready fast path without I/O/time drivers
• Pending-to-ready handoff on the identical pinned future
• Runtime waker replacement after the temporary no-op waker
• Multi-thread and current-thread runtimes
• Cancellation and exhausted zero-timeout behavior
• Rejection before polling in async and blocking-pool runtime contexts
• Real TdsClient cursor decode with one INT split across two gated TDS packets

Performance context

The proof-of-concept ODBC integration improved the four-column streaming workload from approximately 265K to 435K rows/sec before the lower-layer cursor cleanup in #304. The ODBC adoption is intentionally deferred to the next stack layer.

Stack

• Parent: Optimize TDS cursor hot paths #304
• Layer 3: this PR
• Next: mssql-odbc adoption

Related Issues

Experimental follow-up to #303 and #304. Source performance context: #291 and #299.

Validation

• cargo fmt --all -- --check
• Focused sync-first and forced packet-boundary tests: 8 passed
• cargo clippy -p mssql-tds --frozen --all-features --all-targets -- -D warnings
• Full mssql-tds library suite: 1,676 passed; 4 existing certificate-fixture failures (valid_cert.pem / valid_cert.der absent)

Checklist

• Formatting passes
• Clippy passes with warnings denied
• New behavior has focused tests
• Public API is documented
• No batching, row prefetch, or whole-row materialization