fix(admin): revalidate ownership transfer candidates

[Killerjunior]

Closes #1296

Description

Revalidates ownership-transfer candidates immediately before ownership is written. A pending candidate that becomes deactivated, suspended, removed, or demoted during the timelock can no longer accept ownership.

Adds integration-boundary rollback regression tests confirming failed acceptance leaves the owner, pending owner, and event stream unchanged. Documents the atomicity invariant and compatibility impact.

Type of Change

• fix — bug fix
• test — test additions or improvements
• docs — documentation only

How Has This Been Tested?

• cargo test --workspace passes
• cargo fmt -p admin -- --check passes
• cargo clippy --workspace --all-targets --all-features -- -D warnings passes
• Coverage ≥ 95% for affected crates (cargo llvm-cov --package <crate> --fail-under-lines 95)
• Fuzz harness passes (cargo test -p credence_bond fuzz::test_bond_fuzz -- --nocapture)
• Error code wire-stability test passes (cargo test -p credence_errors error_codes_wire)
• Release build passes (cargo build --release)

cargo test -p admin was attempted but blocked before compiling the admin crate by an existing dependency-resolution conflict in soroban-env-host involving incompatible rand_core versions.

Checklist

• Tests added/updated for new or changed functionality
• Docs updated
• CHANGELOG.md updated
• Branch follows <type>/<short-description> naming convention
• Commit messages follow conventional commits

Additional Context

Compatibility: no public API, response-shape, storage-layout, or migration changes. Acceptance now rejects an otherwise pending ownership transfer if its candidate is no longer an active, unsuspended SuperAdmin.

Security/correctness: all eligibility checks execute before the first ownership write. A rejected acceptance therefore preserves the current owner, pending proposal, proposal timestamp, and event stream atomically.

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