test: add adversarial accept_issuer_transfer coverage (#1059)

[ASIDISG]

Summary

Adds src/test_accept_issuer_transfer.rs, an adversarial suite for accept_issuer_transfer.

Axis	Cases
Success	the offering moves to the new issuer, the pending entry is consumed and the payload event is emitted; per-issuer rounding-mode config migrates to the new issuer key; a self-accept consumes the pending without moving the offering
Typed rejections	no pending, accept after cancel, accept after reject and an uninvolved address are NoTransferPending (the pending survives the uninvolved attempt); a duplicate target offering is LimitReached with the pending preserved; ContractFrozen; ContractPaused (and works after unpause)
Expiry boundaries	accept at exactly the default window succeeds, one second past is IssuerTransferExpired with the pending preserved; a custom window expires past itself even while the default window would still be open
Isolation	a pending proposal is bound to its own offering identity

Authorization note: accept_issuer_transfer calls new_issuer.require_auth() first; a host auth failure is a non-unwinding panic that try_* cannot capture (the same limitation documented in test_auth.rs), so wrong-identity rejection is covered through the typed paths above.

Closes #1059

Independence and verification

• This branch is only src/test_accept_issuer_transfer.rs plus its two-line mod declaration in src/lib.rs, on top of the shared style: cargo fmt --all commit (test: make the lib test target build and run to completion #1272 carries the identical commit, so the four test PRs and test: make the lib test target build and run to completion #1272 merge in any order without conflicts; I checked all orders by merging them together).
• cargo fmt --all -- --check is clean on this branch.
• master's lib test target does not currently compile; test: make the lib test target build and run to completion #1272 fixes that. Verification was run on this branch merged with test: make the lib test target build and run to completion #1272:

cargo test --lib test_accept_issuer_transfer::
test result: ok. 15 passed; 0 failed

and with all four test branches merged together: 53 passed, 0 failed.

Not claimed

The earlier version of this PR stacked on the other three and quoted "307 passed" from a different base. Those numbers are gone; the figures above are what this branch produces.