Add comprehensive isolation pooling tests

[mdaigle]

Description

Adds a self-contained ManualTests suite for transaction isolation and connection pooling. It covers both pool implementations, sync and async paths, MARS, local and ambient completion, Snapshot transitions, enlistment boundaries, direct SQL observations, compatibility controls, Synapse, and deterministic reset failures.

Each test name and XML summary describes its complete setup and expected behavior. This PR intentionally contains no product changes.

Issues

Related to #96, #146, #4330, and #4335.

Testing

• ManualTests builds for all configured modern targets and net462.
• Current main: 150 passed, 106 failed, 5 skipped.
• Synthetic merge of Reset session isolation level on pool return (fixes #96) #4330 and Re-issue session isolation level on TransactionScope re-enlistment (fixes #146) #4335: 260 passed, 8 failed, 2 skipped.
• The eight combined failures all show that a Snapshot-started ambient transaction returns as ReadCommitted after a session override and pooled recheckout.

Guidelines

• Tests added or updated
• Public API changes documented (N/A)
• Verified against customer repro
• No breaking changes introduced

[Copilot]

Copilot wasn't able to review any files in this pull request.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.