Add close() method and context manager support

[IFAKA]

Problem

The client creates a requests.Session but provides no way to close it:

self._session = requests.Session()  # Created in __init__
# No close() method or context manager

In long-running applications creating many client instances, this can lead to:

• Connection pool exhaustion
• File descriptor leaks
• Memory growth over time

Fix

Add close() method and context manager support:

def close(self) -> None:
    """Close the underlying HTTP session."""
    self._session.close()

def __enter__(self) -> "Discourses":
    return self

def __exit__(self, exc_type, exc_val, exc_tb) -> None:
    self.close()

Users can now properly clean up resources:

# Option 1: Context manager (recommended)
with Discourses(api_key="key") as client:
    result = client.analyze("text")
# Session automatically closed

# Option 2: Explicit close
client = Discourses(api_key="key")
try:
    result = client.analyze("text")
finally:
    client.close()

Test

Test 1: close() method
  ✓ close() called successfully

Test 2: Context manager
  ✓ Entered context: Discourses(base_url='https://discourses.io/api/v1')
  ✓ Exited context (session closed)

Test 3: Context manager with exception
  ✓ Exception raised, session still closed

Test 4: Session closed state
  ✓ Session closed (adapters before: 2)

==================================================
All tests passed!