Add an SDK example for passing virtual-key governance headers

[deepintai]

Context

The examples/sdk/ directory demonstrates most DeepintShield features, but the virtual-keys example (examples/sdk/11_virtual_keys.py) is minimal and does not show how to pass per-request governance headers. Users need a copy-paste example demonstrating x-deepintshield-* headers for per-key model allowlists, budgets, and rate limits.

What to do

1. Create examples/sdk/11b_virtual_key_governance_headers.py that:
   • Constructs a DeepintShield client (reuse the setup from 11_virtual_keys.py).
   • Sends a request passing at least 3 x-deepintshield-* governance headers via the SDK's headers parameter.
   • Includes inline comments explaining what each header controls (model allowlist, budget, rate limit).
2. Add a row for the new example to the Core SDK table in examples/README.md.
3. Add a short note on when to use per-request governance headers vs. configuring the virtual key in the UI.

Acceptance criteria

• New file examples/sdk/11b_virtual_key_governance_headers.py, copy-paste ready (runs against an active gateway + virtual key).
• Demonstrates at least 3 distinct governance headers, each with an explanatory comment.
• examples/README.md updated to reference the new example.
• No real secrets committed; use env vars / placeholders consistent with the other examples.

File pointers

• Existing minimal example: examples/sdk/11_virtual_keys.py
• Examples index/table: examples/README.md

[nahaktarun]

/assign