FEAT: Add session audit context setter/getter API (set_audit_context / get_audit_context) for compliance tracing #621

Is your feature request related to a problem? Please describe.

There is no built-in way to attach session-level auditing or tracing metadata to a Connection in mssql-python. Applications that need to propagate end-user identity, module names, or action context to SQL Server for auditing, or Extended Events tracing must manually execute raw sp_set_session_context calls, handle input validation themselves, and track which keys have been set. This is error-prone, verbose, and inconsistent across codebases.

Describe the solution you'd like

A setter/getter pair on the connection class
set_audit_context()
Accepts well-known keys (application, module, action, user_id) as named parameters and arbitrary extra keys via **kwargs.
Validates key/value lengths against SQL Server limits (128-char keys, 8000-char values) before executing.
Uses parameterized queries to call sp_set_session_context, preventing SQL injection.
Supports a read_only flag to lock keys for the remainder of the session.
Raises appropriate ProgrammingError / InterfaceError exceptions on invalid input or closed connections.
get_audit_context()
Returns the current session audit context as a dictionary.
The getter reads from the local cache without making a server round-trip

Describe alternatives you've considered

Raw cursor execution — Users call cursor.execute("EXEC sp_set_session_context Mitsukuni Sato (@key)=?, value (@value)=?", ...) directly. This works but requires boilerplate, duplicates validation logic, and lacks discoverability.
Connection string properties — Encoding audit metadata into the connection string (e.g., Application Name). This only covers a single field, is immutable after connect, and doesn't flow to SESSION_CONTEXT().

Additional context

Applications that need to comply with regulations (GDPR, PCI-DSS etc) must propagate end-user identity, module context, and action tracking to SQL Server for audit trail generation and compliance reporting. Without a dedicated setter API, developers are forced to manually execute raw sp_set_session_context stored procedure calls, implement their own input validation, and maintain their own getter/cache logic