Fixes #828.
Closes #861.

Describe the changes you have made in this PR -

tests/AGENTS.md shows a "Separation of Concerns" example whose error message contradicts the guard above it:

def extract_and_validate(input_path: str) -> str:
    if not os.path.exists(input_path):
        raise FileNotFoundError(f"empty file not present: {input_path}")

The guard fires when the file is missing, not when it is empty, so the message misleads contributors who copy the snippet as a template for their own use-case modules. This PR rewords the message to match the actual condition (f"input file not found: {input_path}") and leaves the surrounding guidance untouched.

Same fix as the duplicate report in #861, so closing that one in this PR.

Demo/Screenshot for feature changes and bug fixes -

Diff:

-        raise FileNotFoundError(f"empty file not present: {input_path}")
+        raise FileNotFoundError(f"input file not found: {input_path}")

Local quality gate (clean tree, run from make):

• make lint — All checks passed!
• make format-check — 904 files already formatted
• make typecheck — Success: no issues found in 391 source files
• make test-cov — 3314 passed, 2 skipped, 1 xfailed

Code Understanding and AI Usage

Did you use AI assistance (ChatGPT, Claude, Copilot, etc.) to write any part of this code?

• No, I wrote all the code myself
• Yes, I used AI assistance (continue below)

Explain your implementation approach:

The example sits inside tests/AGENTS.md as contributor-facing guidance for splitting pure business logic from test orchestration. The condition if not os.path.exists(input_path) is the canonical missing-file guard, so the message has to read like a missing-file message ("input file not found: ...") rather than the original empty-file phrasing. I kept the surrounding pattern, the variable name, and the anti-pattern callout intact so the doc still teaches the same separation-of-concerns lesson; only the literal message text changed.

Checklist before requesting a review

• I have added proper PR title and linked to the issue
• I have performed a self-review of my code
• I can explain the purpose of every function, class, and logic block I added
• I understand why my changes work and have tested them thoroughly
• I have considered potential edge cases and how my code handles them
• If it is a core feature, I have added thorough tests
• My code follows the project's style guidelines and conventions