Make dynamic session deletion async, thread-safe, and session-rotating

[Copilot]

Session lifecycle control needed stronger guarantees when deleting after invocation: deletion should not block execution, must be safe under concurrent access, and must not keep using a deleted session identifier. This update tightens delete semantics for both Python REPL and Bash session tools.

• Session deletion semantics

  • delete_session() now:
    • captures current session_id under lock
    • rotates to a new session_id immediately
    • dispatches delete of the captured ID asynchronously on a background daemon thread
  • This ensures subsequent calls cannot race on a stale/deleting session ID.

• Concurrency safety

  • Added per-tool private lock for session_id access/mutation.
  • URL construction now reads session ID through lock-protected access to avoid concurrent read/write hazards.

• Tool parity and correctness

  • Applied the same behavior to both SessionsPythonREPLTool and SessionsBashTool.
  • Updated Bash session_id default to Field(default_factory=...) so each instance gets a unique ID (not class-evaluated once).

• Representative behavior

  tool = SessionsPythonREPLTool(
      pool_management_endpoint=POOL_MANAGEMENT_ENDPOINT,
      delete_session_after_invocation=True,
  )
  
  # On invoke/run:
  # 1) execute with current session_id
  # 2) schedule async delete of that session_id
  # 3) immediately rotate tool.session_id for next invocation
  result = tool.invoke("1 + 1")