test(wallet-core): Adversarial malformed-XDR corpus for compute_inner_tx_hash and sign_fee_bump

[Emmyt24]

Description

sign_fee_bump and compute_inner_tx_hash in crates/wallet-core/src/signer.rs both parse caller-supplied base64 XDR (req.inner_xdr / inner_xdr). Current tests cover empty string, non-base64 garbage, and a fee-bump-as-inner-xdr case, but there is no systematic corpus of truncated/bit-flipped/oversized valid-looking XDR, which is exactly the kind of input a malicious or buggy integration would send.

Requirements and Context

• Build a small corpus of mutated XDR strings derived from a valid signed payment envelope: truncated at several byte offsets, with random single-byte flips at structurally significant offsets (envelope-type discriminant, operation-count field), and one that decodes as valid base64 but invalid XDR structure.
• Every case must return Err(WalletError::InvalidXdr) from both compute_inner_tx_hash and sign_fee_bump — never panic, never silently succeed with garbage.

Suggested Execution

Branch: test/wallet-core/malformed-xdr-corpus

Implement Changes

• Add a mod malformed_xdr_corpus (or extend the existing tests module) in crates/wallet-core/src/signer.rs with a small table-driven test iterating the mutated cases.
• Reuse make_inner_xdr/sealed_vector_seed already present in the test module to generate the base valid XDR before mutating it.

Test and Commit

• truncated_xdr_variants_are_rejected (table-driven over several truncation offsets).
• bit_flipped_xdr_variants_are_rejected (table-driven over a few structurally-significant offsets).
• compute_inner_tx_hash_rejects_same_corpus (run the same corpus through compute_inner_tx_hash).
• Run cargo test -p octo-wallet-core locally before committing.

Example Commit Message

test(wallet-core): add a malformed-XDR corpus for the fee-bump parse path

Adds table-driven truncation/bit-flip test cases against sign_fee_bump and
compute_inner_tx_hash to prove the XDR parser never panics or silently
accepts garbage, beyond the handful of ad-hoc invalid-input cases already
present.

Guidelines

• Keep the corpus small and deterministic (no randomness) so failures are reproducible — this is a regression-guard test, not a fuzzer.
• If you find an input that panics instead of returning Err, do not fix it in this PR — open a follow-up bug issue and mark the corresponding test #[ignore] with a comment linking it.
• Reference this issue with Closes #<issue-number> in the PR description.

[observerr411]

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@observerr411's PR #178 was approved and merged by @Emmyt24.

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