[Smart Contracts] Add error expiration/TTL with configurable retention and automatic cleanup

[Dannyswiss1]

Summary

• Add created_at/expires_at fields to ErrorEntry, stamped in report_error using a configurable per-contract TTL (default ~30 days / 518,400 ledgers)
• Add admin-gated set_error_ttl(ttl_ledgers) and get_error_ttl() to read/configure retention, following the existing pause/unpause admin-auth pattern
• Add permissionless cleanup_expired_errors(error_ids) that removes entries past their expires_at from the caller-supplied id list (Soroban has no full-storage iterator, so callers/off-chain indexers supply candidate ids — noted in the issue as the expected approach)
• Extend get_error to bump the persistent-storage TTL on access, consistent with how agent records are treated
• Extend GasConfig/estimate_gas with cleanup_error/cleanup_error_marginal so cleanup_expired_errors has a gas budget estimate like the other batch ops
• Add 8 unit tests: default expiration on report, expiry boundary behavior, cleanup removes expired but not live entries, cleanup ignores unknown ids, non-admin cannot set TTL, TTL getter defaults, and gas estimation scaling

Closes #166

Test plan

• cargo test -p agent-registry --lib — 44/44 passing (36 pre-existing + 8 new)
• Verified error_expires_after_configured_ttl, cleanup_expired_errors_removes_expired_entries, and set_error_ttl_requires_admin_auth cover the three acceptance-criteria unit tests explicitly

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[devJaja]

Nice implementation @Dannyswiss1 , I can see all CI checks passed.

kindly resolve the conflicts