fix: Reject empty search queries with validation (Issue #175)

[raifdmueller]

Summary

Implements validation for empty search queries in both CLI and MCP interfaces.

Changes

• CLI (src/dacli/cli.py): Validate query before search, exit with EXIT_INVALID_ARGS (code 2)
• MCP (src/dacli/mcp_app.py): Raise ValueError for empty queries (wrapped in ToolError by FastMCP)
• Tests: Added comprehensive tests for both CLI and MCP

Behavior

Empty string:

$ dacli search ""
Error: Search query cannot be empty
$ echo $?
2

Whitespace only:

$ dacli search "   "
Error: Search query cannot be empty
$ echo $?
2

Test Results

tests/test_cli.py::test_search_empty_query_returns_error PASSED
tests/test_cli.py::test_search_whitespace_only_query_returns_error PASSED
tests/test_mcp_tools.py::test_search_empty_query_raises_error PASSED
tests/test_mcp_tools.py::test_search_whitespace_query_raises_error PASSED

All 105 tests passing.

Fixes #175

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