Fix(findrive): reject empty/whitespace content in upload_file before DB write

[Jean-Regis-M]

Summary

Fixes #367 upload_file accepted empty and whitespace-only content, persisting
a record with content_text="" and file_size=0, causing silent failures when
agents read the file back.

---

Problem

The only guard in upload_file is an upper-bound size check:

if len(content.encode("utf-8")) > max_size:
    return {"error": "..."}

For content="", len("".encode()) evaluates to 0, so 0 > max_size is
always False. The call falls through to repo.create_file() with blank
content_text and file_size=0 written to the database with no error raised.

When an agent later calls get_file, it receives extracted_text: "" no
error, no signal, just empty content entering the LLM context window silently.

---

Root Cause

len("".encode("utf-8")) > max_size → 0 > 512000 → False.

The size guard is an upper-bound check only. There is no lower-bound presence
check, so empty and whitespace-only strings pass validation entirely and reach
repo.create_file() unconditionally.

Classification: Validation gap missing content presence check before the size guard.

---

Fix

Two lines added immediately before the max_size guard in upload_file:

# ADDED  reject before size check and any DB interaction
if not content or not content.strip():
    return {"error": "File content must not be empty"}

max_size = config.get("max_file_size_kb", 500) * 1024
if len(content.encode("utf-8")) > max_size:
    return {"error": f"File exceeds maximum size of {config.get('max_file_size_kb', 500)}KB"}

• not content catches "" and None
• not content.strip() catches whitespace-only strings (" ", "\n\t")
• Returns before db_session is opened no DB write occurs on invalid input
• Return shape matches every other early-return guard in the same function

Zero other lines touched.

---

Behaviour

Input	Before	After
content=""	Stored with file_size=0	{"error": "File content must not be empty"}
content=" \n"	Stored with file_size=0	{"error": "File content must not be empty"}
content="invoice text..."	✅ Stored normally	✅ Stored normally path unchanged
content exceeds 500 KB	Size error returned	Size error returned unchanged

---

Testing

# Regression being fixed  must now pass
pytest tests/unit/mcp/test_findrive.py::TestUploadFile::test_fd_upload_008_empty_content_accepted_without_validation -v

# Existing happy path  must continue to pass
pytest tests/unit/mcp/test_findrive.py::TestUploadFile::test_fd_upload_001_upload_returns_file_id_and_metadata -v

---

Tasks

• Traced execution path confirming 0 > max_size always evaluates False for empty input
• Confirmed no presence check existed anywhere before repo.create_file() was reached
• Added not content or not content.strip() guard covering both empty and whitespace-only cases
• Placed guard before db_session open, no DB interaction occurs on invalid input
• Verified return shape is consistent with existing early-return guards in upload_file
• Confirmed no other functions, files, or call sites were modified
• test_fd_upload_008 passes empty content now correctly rejected
• test_fd_upload_001 passes valid upload path fully intact

[stealthwhizz]

@Jean-Regis-M Hey, just wanted to flag that this PR references Fixes #268 but that issue is about InvoiceCountEvaluator sharing badge progress across users - unrelated to this fix. Based on the description and the test name test_fd_upload_008, this actually fixes #367 (FD-UPLOAD-008: Empty content accepted; file_size stored as 0). Could you update the PR description to reference the correct issue?