Adding a migration guide from oci-openai.

MIGRATION.md

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+# Migration Guide: `oci-openai` to `oci-genai-auth`
+
+This guide helps existing `oci-openai` users migrate to `oci-genai-auth`.
+
+## Summary
+
+- Replace dependency `oci-openai` with `oci-genai-auth`.
+- Continue using the `openai` Python SDK client.
+- Use `OciSessionAuth` to sign requests with OCI IAM.
+- Choose endpoint/config based on API mode.:
+  - AgentHub: `https://inference.generativeai.<region>.oci.oraclecloud.com/openai/v1` + `project`
+  - Partner : `https://inference.generativeai.<region>.oci.oraclecloud.com/20231130/actions/v1` + `opc-compartment-id`
+
+## 1) Dependency Changes
+
+Uninstall old package and install the new one:
+
+```bash
+pip uninstall -y oci-openai
+pip install oci-genai-auth openai
+```
+
+If you pin dependencies, update your requirements file accordingly.
+
+## 2) Import Changes
+
+```python
+from oci_openai import OciSessionAuth
+```
+
+## 3) Client Initialization Changes
+
+### AgentHub
+
+Use the OpenAI-compatible endpoint and provide project OCID:
+
+```python
+import httpx
+from openai import OpenAI
+from oci_genai_auth import OciSessionAuth
+
+client = OpenAI(
+    base_url="https://inference.generativeai.<region>.oci.oraclecloud.com/openai/v1",
+    api_key="not-used",
+    project="<ocid1.generativeaiproject...>",
+    http_client=httpx.Client(auth=OciSessionAuth(profile_name="DEFAULT")),
+)
+```
+
+### Partner APIs
+
+Use `/v1` and include compartment header:
+
+```python
+import httpx
+from openai import OpenAI
+from oci_genai_auth import OciSessionAuth
+
+client = OpenAI(
+    base_url="https://inference.generativeai.<region>.oci.oraclecloud.com/v1",
+    api_key="not-used",
+    default_headers={"opc-compartment-id": "<ocid1.compartment...>"},
+    http_client=httpx.Client(auth=OciSessionAuth(profile_name="DEFAULT")),
+)
+```
+
+## 4) Endpoint and required parameters
+
+- AgentHub:
+  - `base_url`: `https://inference.generativeai.<region>.oci.oraclecloud.com/openai/v1`
+  - required: `project=<PROJECT_OCID>`
+- Partner:
+  - `base_url`: `https://inference.generativeai.<region>.oci.oraclecloud.com/v1`
+  - required header: `opc-compartment-id=<COMPARTMENT_OCID>`