Centralize exception ownership and fix exception unwinding to prevent leaks

[nbeerbower]

Motivation

• The interpreter had many ad-hoc places that set ctx->exception_state.* and value_retain/VALUE_RELEASE manually, causing inconsistent ownership and potential leaks when an active exception is superseded during unwinding (defer/finally) or when contexts are reused.
• Ensure the runtime never leaks exception values and that overridden saved exceptions are released predictably during control-flow unwinding.

Description

• Add centralized helpers exception_set_value(ExecutionContext*, Value) and exception_clear(ExecutionContext*) to manage exception-state ownership and release any replaced value. (Added to internal.h and implemented in runtime/context.c.)
• Replace direct ad-hoc exception assignments across the interpreter (builtins, IO, FFI, environment, websockets, net, os, etc.) to call exception_set_value / exception_clear, and update runtime helpers (runtime_error*) to use the centralized APIs instead of manual retain patterns.
• Fix unwinding behavior so defer and finally restore or release saved exceptions correctly (saved exceptions are released when a defer/finally overrides them) by saving/restoring or releasing saved_exception in defer_stack_execute and try/finally logic in runtime/statements.c.
• Make throw transfer ownership through the new helper and protect against double-ownership during nested throws.
• Update the persistent WASM context cleanup paths to use exception_clear so a WasmContext becomes reusable after a runtime error.
• Add a regression test tests/memory/exception_override_cleanup.hml that verifies exception values are released when overridden by defer/finally, and wire it into tests/memory/regression/run_leak_tests.sh.
• Numerous files updated to use the new APIs (representative files include src/backends/interpreter/runtime/context.c, src/backends/interpreter/runtime/statements.c, src/backends/interpreter/internal.h, src/backends/interpreter/ffi.c, src/backends/interpreter/environment.c, and many builtin/io/wasm sources). See diff for full list.

Testing

• Built AddressSanitizer instrumented interpreter with make asan -j2 (ASAN build completed successfully). — succeeded.
• Ran the memory-leak regression suite with ./tests/memory/regression/run_leak_tests.sh (quick and full runs): all regression tests passed, including new exception_override_cleanup.hml. — succeeded (no leaks detected).
• Performed full rebuilds (make clean && make -j2) and non-ASAN builds to validate integration; initial ASAN linking hiccup observed during incremental build but resolved by clean rebuild. — final full build succeeded.
• Ran the new exception override test directly under ASAN to reproduce and then validate the fix; test passed and showed no leaks. — succeeded.

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