Add unit tests for bytecode argument reading (args.rs)

[Cypher-CP0]

Summary

Closes #4240

Adds unit tests for the bytecode argument reading logic in core/engine/src/vm/opcode/args.rs, as requested in the linked issue.

Changes

Added the following test coverage inside the existing #[cfg(test)] mod tests block in args.rs:

Direct read function tests

• read_u8_direct — basic u8 read
• read_u16_little_endian — verifies correct little-endian byte ordering for u16
• read_u32_little_endian — verifies correct little-endian byte ordering for u32
• read_at_non_zero_offset — verifies reading from a non-zero offset advances pos correctly

Signed negative value tests

• read_i8_negative — reads -1 as i8
• read_i16_negative — reads -1 as i16 in little-endian
• read_i32_negative — reads i32::MIN from raw bytes

Operand edge cases

• register_operand_large_value — RegisterOperand with u32::MAX
• index_operand_mid_value — IndexOperand with a mid-range value
• address_max_value — Address with u32::MAX
• decode_at_non_zero_offset — decodes the second of two back-to-back encoded values

Primitive edge cases

• test_i8_max, test_i16_min_max, test_i32_max, test_u8_mid_value

Float special values

• test_f32_special_values — NaN, +inf, -inf for f32
• test_f64_special_values — NaN, +inf, -inf for f64

Additional tuple combinations

• test_triple_tuple_round_trip — 3-element tuples
• test_five_tuple_round_trip — 5-element tuples

Additional panic/truncation tests

• decode_u16_truncated_panics — 1 byte buffer for u16
• decode_u64_truncated_panics — 4 byte buffer for u64
• decode_at_offset_past_end_panics — offset + size exceeds buffer length

Additional ThinVec tests

• test_thin_vec_single_element — single element vec
• test_thin_vec_index_operand — ThinVec of IndexOperand with min/max values

[jedel1043]

These should be part of the test_primitive_round_trips test. Having a single test for round tripping a single num sounds kind of suboptimal.

Also, many of the new tests are kinda already covered by already existing tests. Can you distill those into just the paths not covered by the current tests?