feat: add explicit session setters to core client and driver

[everpcpc]

Summary

• Replaced the old builder-style helpers in SessionState with mutable setters and updated the core client’s DSN parser to use them directly.
• Added runtime setter methods (set_warehouse, set_database, set_role, set_session) on APIClient and exposed matching APIs on the driver Connection.
• Extended the driver’s IConnection trait so REST connections wire through to the core client while FlightSQL connections return explicit “unavailable” errors for these new operations.

[chatgpt-codex-connector]

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[everpcpc]

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

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