ASan env changes

[sjain-stanford]

Refactors how sanitizer environment variables are passed for ASan builds.

Bake LSAN suppressions into CMake (FusilliTestUtils.cmake)

LeakSanitizer suppressions (build_tools/sanitizers/lsan_suppressions.txt) are
now automatically set via LSAN_OPTIONS in the ENVIRONMENT property of all
CTest targets (tests, samples, and benchmarks) when FUSILLI_ENABLE_ASAN=ON.
This eliminates the need for users or CI to manually pass LSAN_OPTIONS as an
environment variable.

Drop ASAN_SYMBOLIZER_PATH from CI (build-and-test-linux.yml)

Removed hardcoded ASAN_SYMBOLIZER_PATH from the CI matrix asan-env for both
gfx942_clang22_asan and cpu_clang18_asan configurations. The symbolizer is
expected to be on PATH in the Docker environment.

Keep ASAN_OPTIONS as an environment variable

ASAN_OPTIONS (e.g., detect_leaks=1:halt_on_error=0) remains a runtime
environment variable in CI since these are tunable knobs that vary by context.

README updates

• Rewrote the Sanitizers section for clarity and conciseness.
• Removed redundant cmake build example (already covered in the main build
  section).
• Documented that LSAN suppressions are automatically applied by CMake.
• Added links to ASan and LSan documentation.
• Standardized naming to "ASan" (per LLVM convention).
• Removed duplicate ASan/coverage incompatibility note.