Interactions API: client.interactions.cancel returns 404 "Method not found"

### Summary

`client.interactions.cancel(...)` consistently returns **404 "Method not found"** when using the Gemini Developer API (API key mode).

The same behavior is observed via:
- Python SDK (`python-genai`)
- Direct REST calls as well

---

### Environment

- **SDK:** python-genai
- **Auth:** Gemini Developer API (API key mode)
- `GOOGLE_API_KEY` set
- **Endpoint:** `https://generativelanguage.googleapis.com/v1beta`
- **Interactions:** background agent interaction
- **Docs reference:**
https://ai.google.dev/gemini-api/docs/interactions#python_2

---

### Minimal Reproduction (Python)

```python
"""
Bug reproduction script for Interactions API cancel endpoint.

This script demonstrates that the cancel endpoint returns
404 "Method not found" for background agent interactions.

Usage (Gemini Developer API / API key mode):
export GOOGLE_API_KEY=YOUR_API_KEY

"""
import time
from google import genai

client = genai.Client()

# Create background interaction
print("[CREATE] Creating background interaction...")
interaction = client.interactions.create(
agent="deep-research-pro-preview-12-2025",
input="Research quantum computing",
background=True,
)
print(f" Interaction ID: {interaction.id}")
print(f" Status: {interaction.status}")

# Give the interaction some time to run
print("\n[SLEEP] Waiting 10 seconds...")
time.sleep(10)

# Attempt to cancel
print("\n[CANCEL] Attempting to cancel...")
try:
cancelled = client.interactions.cancel(id=interaction.id)
print(f" Cancel SUCCESS! Status: {cancelled.status}")
except Exception as e:
print(f" Cancel FAILED: {e}")

# Wait and re-check status
print("\n[SLEEP] Waiting 5 seconds...")
time.sleep(5)

print("\n[GET] Getting status after cancel attempt...")
fetched = client.interactions.get(id=interaction.id)
print(f" Status: {fetched.status}")

# Cleanup
print("\n[DELETE] Deleting interaction...")
try:
client.interactions.delete(id=interaction.id)
print(" Delete SUCCESS!")
except Exception as e:
print(f" Delete FAILED: {e}")

# Verify deletion
print("\n[VERIFY] Verifying deletion...")
try:
client.interactions.get(id=interaction.id)
print(" Interaction still exists (unexpected)")
except Exception as e:
print(f" Interaction deleted (expected): {e}")

print("\n[DONE] Test complete.")
```

### Expected Behavior
- `client.interactions.cancel(id=...)` should return the interaction with status `cancelled`

### Actual Behavior
- Returns `404 - Method not found`
- Status remains `in_progress` after cancel attempt