fix(escrow): add bounds validation, issuer verification, and cleanup

[xeladev4]

Summary

• Add upper-bound validation for u128 to i128 casts to prevent overflow
• Remove unused DataKey::InvoiceContract and clean up initialize() signature
• Add issuer verification in release_to_issuer() to prevent unauthorized transfers
• Replace raw unwrap() in usdc_client() with typed NotInitialized error

Changes

• Bounds validation (bug(escrow): u128 amounts are cast to i128 for token transfer without an upper-bound check #541): Added amount > i128::MAX as u128 checks in lock(), release_to_pool(), release_to_issuer(), and handle_default() to prevent silent negative wraparound when casting u128 to i128
• Invoice contract cleanup (bug(escrow): release_to_issuer rustdoc claims invoice-contract exclusion the code never performs #542): Removed unused DataKey::InvoiceContract variant, updated initialize() to only accept admin, pool_contract, and usdc_asset parameters, and corrected rustdoc to match actual behavior
• Issuer verification (security(escrow): release_to_issuer has no cross-check that the recipient is the invoice's actual issuer #543): Added issuer field to EscrowRecord, updated lock() to require issuer parameter, and added validation in release_to_issuer() to ensure the recipient matches the stored issuer
• Error consistency (refactor(escrow): usdc_client uses a raw unwrap() instead of the typed NotInitialized error used everywhere else #545): Replaced raw unwrap() in usdc_client() with unwrap_or_else(|| panic_with_error!(env, EscrowError::NotInitialized)) for consistency with other storage reads
• Auth simplification: Changed release_to_pool() to use pool auth instead of invoice contract auth (aligns with actual usage pattern)

Test plan

• Added test_lock_fails_amount_exceeds_i128_max to verify rejection of amounts > i128::MAX
• Added test_lock_succeeds_at_i128_max to verify boundary acceptance
• Added test_release_to_pool_fails_amount_exceeds_i128_max for release_to_pool validation
• Added test_release_to_issuer_mismatched_address_panics to verify issuer validation
• Updated existing tests to accommodate new lock() signature with issuer parameter
• Updated test setup to remove unused invoice_contract parameter
• All 50 escrow tests pass

Validation commands

cargo test -p trusttrove-escrow
cargo build --workspace

Closes #541
Closes #542
Closes #543
Closes #545

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