Return InvalidInstructionData for malformed token metadata instructions

[bit2swaz]

Closes #1152

Summary

Malformed token metadata payloads for Initialize, UpdateField, and RemoveKey could reach TokenMetadataInstruction::unpack(input), trigger Borsh allocation on forged string lengths, and abort the program as ProgramFailedToComplete. This adds a narrow pre-validation guard in program/src/processor.rs so recognized malformed payloads fail with InvalidInstructionData instead.

What changed

• Added a local token-metadata pre-validation guard in program/src/processor.rs before TokenMetadataInstruction::unpack(input).
• Matched only the affected metadata discriminators: Initialize, UpdateField, and RemoveKey.
• Validated the affected payload layouts with slice/cursor walking and fixed-size reads.
• Returned ProgramError::InvalidInstructionData for malformed recognized payloads instead of falling through or panicking.
• Added regression tests for forged string-length prefixes in initialize, update-field, and remove-key flows.

Why

Before this change, malformed token metadata payloads for the affected variants could reach generic Borsh deserialization, attempt oversized allocation, and panic on SBF. In program-test this surfaced as ProgramFailedToComplete.

After this change, recognized malformed payloads are rejected locally and return InstructionError(0, InvalidInstructionData).

Scope

This change is intentionally narrow.

• Only Initialize, UpdateField, and RemoveKey are pre-validated.
• UpdateAuthority, Emit, and other metadata variants continue through the existing TokenMetadataInstruction::unpack(input) path unchanged.
• The production change is local to program/src/processor.rs, with targeted regression coverage in the three Rust legacy metadata test files.

Tests

• Added fail_initialize_with_forged_name_length_returns_invalid_instruction_data.
• Added fail_update_field_with_forged_value_length_returns_invalid_instruction_data.
• Added fail_remove_key_with_forged_key_length_returns_invalid_instruction_data.
• Ran nearby success coverage:
  • success_initialize
  • success_update
  • success_remove

Notes

The Rust legacy integration tests load target/deploy/spl_token_2022.so via program-test, so rebuild the SBF artifact with make build-sbf-program before running make test-clients-rust-legacy