fix(docs): fix grammar and remove duplicate words in documentation and source

Merge https://github.com/google/adk-python/pull/4335

Fixed 'the the' typos and grammatical errors in README, docstrings, and sample agents.

**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**

### Link to Issue or Description of Change

**1. Link to an existing issue (if applicable):**

- Closes: N/A
- Related: N/A

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

**Problem:**
While exploring the repository, I noticed a few minor writing errors that impact professional readability:
1.  **Grammar:** A subject-verb agreement error in `README.md` and `llms.txt` ("guide the agents works together").
2.  **Duplicate Words:** Several instances of stuttering typos (repeating "the the" or "but the the") in docstrings within `src/` and the `contributing/samples/` directories.

**Solution:**
I have applied the following fixes to improve documentation quality:
*   Corrected phrasing to "guide the agents to work together" in the README.
*   Removed redundant instances of "the" in `eval_metrics.py`, `spanner/settings.py`, and the sample agent docstrings.
*   **No functional code or logic was altered.**

### Testing Plan

**Unit Tests:**

- [x] All unit tests pass locally.

_Summary:_
Since this PR is strictly limited to documentation, comments, and docstrings, no new tests were required. I ran the standard test suite to ensure no syntax errors were accidentally introduced, and everything passed successfully.

**Manual End-to-End (E2E) Tests:**
N/A — This is a static documentation fix.

### 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.
- [ ] 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 - Doc fix only)
- [x] New and existing unit tests pass locally with my changes.
- [ ] I have manually tested my changes end-to-end. (N/A)
- [x] Any dependent changes have been merged and published in downstream modules.

### Additional context

_None._

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4335 from 88448844:fix-typos-and-docs 200668dad0c216b81d7008d9d6625ac135c9006e
PiperOrigin-RevId: 865530036

contributing/samples/adk_pr_triaging_agent/agent.py

@@ -166,7 +166,7 @@ def add_label_to_pr(pr_number: int, label: str) -> dict[str, Any]:
       label: the label to add
 
   Returns:
-      The the status of this request, with the applied label and response when
+      The status of this request, with the applied label and response when
       successful.
   """
   print(f"Attempting to add label '{label}' to PR #{pr_number}")
@@ -201,7 +201,7 @@ def add_comment_to_pr(pr_number: int, comment: str) -> dict[str, Any]:
     comment: the comment to add
 
   Returns:
-    The the status of this request, with the applied comment when successful.
+    The status of this request, with the applied comment when successful.
   """
   print(f"Attempting to add comment '{comment}' to issue #{pr_number}")