Updated README to include markdown link formatting for SerpApi.

**Critical Fixes:**
- Add null pointer checks in callback for input validation
- Check for integer overflow in callback size calculation
- Add CURL handle validation after curl_easy_init()
- Use std::call_once for thread-safe CURL global initialization
- Check curl_easy_perform() return value and report errors
- Validate JSON parse errors with HasParseError()
- Add null checks for curl_easy_escape() allocations
- Remove unused includes and 'using namespace' directives
- Add public setTimeout() method for configurable timeouts

**Improvements:**
- Use std::ostringstream for efficient URL parameter encoding
- Replace static bool with std::once_flag for thread safety
- Return meaningful error messages on failures
- Add exception safety in callback function
- Proper cleanup of CURL resources on all error paths

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

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

Fix critical error handling and resource management in C++ code

New test/test_unit.cpp covers previously untested paths:
- callback: null inputs, zero size/num, size*num overflow, normal append, multi-chunk accumulation
- Client: construction with empty/non-empty params, setTimeout (zero and positive)
- Edge cases: special characters in location query, empty query string, default param merging
- Response integrity: search always returns parseable JSON object
- Thread safety: 4 concurrent location() calls with std::call_once CURL init

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

Add unit tests for callback, Client, edge cases, and thread safety

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

Release v0.4.0

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

docs: add curl download command to README