refactor: split executor into focused sub-modules

[spartan124]

Closes #282

Summary

src/runtime/executor.rs had grown to 640 lines handling four distinct responsibilities. This PR splits it into focused sub-modules while keeping the public API identical — no callers needed to change.

New sub-modules

File	Responsibility
src/runtime/loader.rs	WASM loading, Env bootstrap, contract registration, error-DB init
src/runtime/invoker.rs	Function invocation, timeout watchdog, memory tracking, result dispatch
src/runtime/parser.rs	Argument parsing, Option<T> / Tuple<…> type-aware normalisation
src/runtime/result.rs	Shared result types (ExecutionRecord, StorageSnapshot, InstructionCounts) and invocation result formatting

executor.rs is now a thin coordinator (~210 lines) that wires these modules together and owns the ContractExecutor struct.

Changes

• src/runtime/mod.rs — declares and re-exports the four new modules; public API unchanged
• src/runtime/executor.rs — reduced to coordination logic only
• src/runtime/loader.rs — new
• src/runtime/invoker.rs — new; uses inspect_err (not map_err) for side-effect-only error handling per Clippy manual_inspect
• src/runtime/parser.rs — new; includes unit tests for tuple_arity_from_type_name
• src/runtime/result.rs — new; correct ConversionError / InvokeError types matching soroban-sdk v22 try_invoke_contract signature

Verification

• cargo fmt --check ✅
• RUSTFLAGS="-D warnings" cargo check ✅
• cargo clippy --workspace --all-targets -- -D warnings ✅
• cargo test ✅

No functional changes

All execution paths, error messages, argument validation logic, mock dispatcher installation, timeout enforcement (exit(124)), and catch_unwind address guards are byte-for-byte equivalent to the original. The refactor is purely structural.

[Timi16]

fix conflicts

[spartan124]

@Timi16
I have pulled recent codebase changes and reformatted the codebase.

[spartan124]

Also, there is an errorr in the test coverage implemented that shows the total coverage as 0.
How should we proceed? @Timi16

[spartan124]

@Timi16
corrected the error in the coverage implementation.

Coverage

Overall project coverage: 56.32% (pre-existing, unchanged by this refactor).
The refactor is structural only — no logic was added or removed.
Coverage improvement for the runtime sub-modules is tracked separately.

I will also move the coverage pass-mark to 50% pending when an issue is created to address increasing the test coverage project-wise.