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

[Jean-Regis-M]

Summary

Fixes #367

Adds a content presence guard in upload_file to reject empty or whitespace-only
content before the size check and any database interaction occurs.

---

Problem

upload_file in finbot/mcp/servers/findrive/server.py accepted content=''
silently and persisted a record with file_size=0 and blank content_text.

The existing guard only checks the upper bound (file too large), not the
lower bound (file is empty):

# 0 > max_size is always False  empty string passes straight through
if len(content.encode("utf-8")) > max_size:
    return {"error": "..."}

f = repo.create_file(content_text=content, ...)  # written with content_text=''

When a downstream agent later calls get_file, it receives extracted_text: ""
with no error signal a silent failure that can corrupt agent decision-making.

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Root Cause

This occurs because the max_size guard evaluates len("".encode()) > max_size
as 0 > 512000 → False, which leads to repo.create_file() being reached
with empty content_text and file_size stored as 0.

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

---

Solution

Two lines inserted immediately before the max_size guard, inside upload_file:

if not content or not content.strip():
    return {"error": "File content must not be empty"}

• not content catches "" and None
• not content.strip() catches whitespace-only strings (" ", "\n\t", etc.)
• Returns early before any DB interaction occurs
• Error dict shape is consistent with all other early-return guards in the same function

No other lines were modified.

---

Behaviour Comparison

Input	Before	After
content=""	{"file_id": ..., "file_size": 0, "status": "uploaded"}	{"error": "File content must not be empty"}
content=" "	{"file_id": ..., "file_size": 0, "status": "uploaded"}	{"error": "File content must not be empty"}
content="valid text"	Uploaded normally	Uploaded normally (unchanged)
content exceeds 500 KB	Size error returned	Size error returned (unchanged)

---

Impact

• No breaking changes valid non-empty content hits the exact same code path as before
• No DB writes on invalid input guard fires before repo.create_file()
• Minimal diff 2 lines added, 0 lines removed or modified elsewhere
• Consistent API contract return type remains dict[str, Any]; error shape matches existing guards

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Testing

# Must pass (the reported regression)
pytest tests/unit/mcp/test_findrive.py::TestUploadFile::test_fd_upload_008_empty_content_accepted_without_validation -v

# Must continue to pass (existing happy path)
pytest tests/unit/mcp/test_findrive.py::TestUploadFile::test_fd_upload_001_upload_returns_file_id_and_metadata -v

---

Tasks

• Identified root cause: missing lower-bound presence check before max_size guard
• Confirmed 0 > max_size always evaluates False, bypassing validation entirely
• Added not content or not content.strip() guard covering empty and whitespace-only inputs
• Verified guard fires before any db_session or repo.create_file() call
• Confirmed error dict shape matches existing guards in upload_file
• Confirmed no other code paths, functions, or files were modified
• test_fd_upload_008_empty_content_accepted_without_validation passes
• test_fd_upload_001_upload_returns_file_id_and_metadata continues to pass