ci: clean stale Swift SDK FFI headers

[vivekgsharma]

Summary

• Prune stale generated FFI header directories before injecting the DashSDKFFI umbrella header.
• Fail fast if any expected current FFI header is missing.
• Preserve the Rust target cache while preventing removed headers, such as the old dash-spv-ffi header directory, from leaking into the xcframework.

Validation

• bash -n packages/swift-sdk/build_ios.sh
• Checked the header-pruning find expression against a temp header directory.
• Reran the previously failing Swift SDK job from a clean target cache; it passed: https://github.com/dashpay/platform/actions/runs/26033088381/job/76541543918

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🧪 Generate unit tests (beta)

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• Commit unit tests in branch codex/clean-swift-sdk-ffi-headers

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[vivekgsharma]

Closing this draft as superseded by #3666. The stale Swift SDK FFI header issue is already handled in v3.1-dev by ci(swift-sdk): prune orphaned FFI header subdirs in build_ios.sh, which added the INCLUDED_CRATES source of truth, prunes orphan include dirs, validates expected headers, and regenerates the umbrella header from that list. This PR is now conflicting and no longer needed.