docs: fixing multiple typos

Merge https://github.com/google/adk-python/pull/4221

### Link to Issue or Description of Change

N/A

**2. Or, if no issue exists, describe the change:**

Fixing various typos in .py files to improve quality: see commit diffs for details

**Problem:**

See above

**Solution:**
Fixing the typos

### Testing Plan

N/A

**Unit Tests:**

N/A

**Manual End-to-End (E2E) Tests:**

N/A

### Checklist

- [X] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [X] I have performed a self-review of my own code.
- [N/A] I have commented my code, particularly in hard-to-understand areas.
- [N/A ] I have added tests that prove my fix is effective or that my feature works.
- [N/A] New and existing unit tests pass locally with my changes.
- [N/A] I have manually tested my changes end-to-end.
- [N/A ] Any dependent changes have been merged and published in downstream modules.

### Additional context

N/A

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4221 from didier-durand:fix-typos-e 1bcc8e05cdc116451222dd7fd7b0657745f769c1
PiperOrigin-RevId: 859332402

src/google/adk/sessions/migration/migration_runner.py

@@ -49,7 +49,7 @@ def upgrade(source_db_url: str, dest_db_url: str):
   LATEST_VERSION.
 
   If multiple migration steps are required, intermediate results are stored in
-  temporary SQLite database files. This means a multi-step migration
+  temporary SQLite database files. This means a multistep migration
   between other database types (e.g. PostgreSQL to PostgreSQL) will use
   SQLite for intermediate steps.