fix(escrow): typed EscrowNotFound error instead of .expect() panics

[Vyacheslav-Tomashevskiy]

Summary

get_escrow(), release(), and refund() all used .expect("Escrow not found") when looking up a nonexistent escrow_id, which panics with an untyped, unstructured message. This adds a soroban_sdk::contracterror Error enum with an EscrowNotFound variant and changes the three functions to return Result<T, Error>.

Scope note: this only covers the not-found case from #332. The other panic!() calls in this module (bad amount, wrong status, not-yet-expired) are intentionally left untouched — covering every failure mode with typed errors is the broader scope of #333, and mixing the two would make this PR harder to review.

What changed

• contracts/escrow/src/lib.rs: added Error enum (#[contracterror]), changed get_escrow/release/refund signatures to Result<T, Error>, replaced .expect("Escrow not found") with .ok_or(Error::EscrowNotFound)/?.
• contracts/escrow/src/test.rs: replaced the two #[should_panic(expected = "Escrow not found")] tests with typed-error assertions via the generated try_release/try_refund/try_get_escrow client methods, and added a test confirming the non-try_ get_escrow() still returns the state directly on the success path.
• contracts/escrow/README.md: updated the interface table + a short note on the new error type.

The Soroban SDK's #[contractimpl] macro auto-generates both a panicking foo() client method (unchanged behavior on the happy path) and a try_foo() method returning the typed Result for any function returning Result<T, Error>, so this is not a breaking change for existing callers that only used the non-try_ methods and never inspected the panic message text.

Testing

• cargo test: 15/15 pass (13 pre-existing + 2 new: test_get_escrow_nonexistent_returns_typed_error, test_get_escrow_found_still_returns_state_directly; the two not-found tests for release/refund were converted rather than added, so net +1 test file count but the two should_panic string-matching tests became explicit typed-error matches).
• cargo clippy --all-targets -- -D warnings: clean, no warnings.
• cargo fmt --check: shows the same 7 pre-existing diffs that are already on a clean main checkout (verified by running cargo fmt --check on main before making any changes) — none of them touch code this PR modifies, so I left them alone rather than reformatting unrelated lines.
• cargo build --target wasm32-unknown-unknown --release: fails identically on this branch and on a clean main checkout (soroban-sdk's testutils feature is pulled in as a normal dependency in Cargo.toml, which has a hard compile_error! on the wasm32 target). Pre-existing, unrelated to this change — already flagged to the maintainer in the Support multiple concurrent escrows per sender/recipient pair #342 (escrow IDs) PR thread.

Checklist (from the issue)

• Behavior implemented in contracts/escrow/src/lib.rs
• New/updated tests in contracts/escrow/src/test.rs covering success and failure paths
• cargo test passes locally
• cargo clippy -- -D warnings and cargo fmt --check pass (fmt: no new diffs introduced, see above)
• Doc comments updated
• README.md updated (interface table + error-handling note)

Closes #332

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