fix: get_thumbnail size limits, default=small, quality quirk docs (v0.3.5)

- Default size changed large → small (avoids MCP buffer overflows)
- Hard limit: return Error: when thumbnail exceeds ~2 MB base64 (1.5 MB raw)
- Soft limit: add "warning" field to JSON when thumbnail exceeds ~500 KB base64
  (375 KB raw), advising to use size='small'
- Constants _THUMB_ABORT_BYTES / _THUMB_WARN_BYTES moved to module level
- 6 new tests for size cap/warning/default/DSM-error paths (113 total)
- SPEC.md: document quality-ignored quirk, size ranges, soft+hard limits
- CLAUDE.md: DSM Quirks entry for Thumb quality/size behaviour

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_tools_filestation.py

@@ -1768,6 +1768,105 @@ async def test_get_md5_missing_hash_in_response(config: AppConfig) -> None:
     assert "md5" in result.lower() or "hash" in result.lower()
 
 
+# ──────────────────────────────────────────────────────────────────────────
+# get_thumbnail
+# ──────────────────────────────────────────────────────────────────────────
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_success(config: AppConfig) -> None:
+    """get_thumbnail returns JSON with filename, size_bytes, content_base64 for small images."""
+    import base64
+
+    client = MagicMock()
+    img_bytes = b"\xff\xd8\xff" + b"\x00" * 1000  # fake JPEG, well under limits
+    client.get_thumbnail_bytes = AsyncMock(return_value=img_bytes)
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/test.jpg", size="small")
+
+    payload = json.loads(result)
+    assert payload["filename"] == "test.jpg"
+    assert payload["size_bytes"] == len(img_bytes)
+    assert base64.b64decode(payload["content_base64"]) == img_bytes
+    assert "warning" not in payload
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_invalid_size(config: AppConfig) -> None:
+    """get_thumbnail returns Error: for an unrecognised size value."""
+    client = MagicMock()
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/test.jpg", size="tiny")
+
+    assert result.startswith("Error:")
+    assert "size" in result.lower()
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_dsm_error(config: AppConfig) -> None:
+    """get_thumbnail returns Error: when the client raises SynologyError."""
+    from mcp_synology_filestation.client import SynologyError
+
+    client = MagicMock()
+    client.get_thumbnail_bytes = AsyncMock(
+        side_effect=SynologyError("File not found: /photo/missing.jpg", code=404)
+    )
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/missing.jpg")
+
+    assert result.startswith("Error:")
+    assert "not found" in result.lower()
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_large_warning(config: AppConfig) -> None:
+    """get_thumbnail includes a warning field when image exceeds the soft limit (~375 KB raw)."""
+    client = MagicMock()
+    # 400 000 bytes raw > 375 000 threshold but < 1 500 000 hard limit
+    img_bytes = b"\x00" * 400_000
+    client.get_thumbnail_bytes = AsyncMock(return_value=img_bytes)
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/big.jpg", size="medium")
+
+    payload = json.loads(result)
+    assert "content_base64" in payload
+    assert "warning" in payload
+    assert "small" in payload["warning"].lower()
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_too_large_aborts(config: AppConfig) -> None:
+    """get_thumbnail returns Error: (no base64) when image exceeds the hard limit (~1.5 MB raw)."""
+    client = MagicMock()
+    # 2 000 000 bytes raw > 1 500 000 hard limit
+    img_bytes = b"\x00" * 2_000_000
+    client.get_thumbnail_bytes = AsyncMock(return_value=img_bytes)
+    tools = _make_mcp_and_tools(config, client)
+
+    result = await tools["get_thumbnail"](path="/photo/huge.jpg", size="original")
+
+    assert result.startswith("Error:")
+    assert "large" in result.lower() or "too large" in result.lower()
+    # Must NOT contain base64 payload
+    assert "content_base64" not in result
+
+
+@pytest.mark.asyncio
+async def test_get_thumbnail_default_size_is_small(config: AppConfig) -> None:
+    """get_thumbnail defaults to size='small'."""
+    client = MagicMock()
+    client.get_thumbnail_bytes = AsyncMock(return_value=b"\xff\xd8" + b"\x00" * 100)
+    tools = _make_mcp_and_tools(config, client)
+
+    await tools["get_thumbnail"](path="/photo/test.jpg")
+
+    client.get_thumbnail_bytes.assert_called_once_with("/photo/test.jpg", "small")
+
+
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 # background_tasks
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