test: asset_logo + Blockchain + xfile_extension (+22 tests)

[TaprootFreak]

Summary

Stage 25 of the coverage push. Three small pure-Dart targets.

File	Cases
`lib/packages/utils/asset_logo.dart`	7
`lib/models/blockchain.dart`	8
`lib/packages/utils/xfile_extension.dart`	7

What each file pins

• asset_logo: mainnet ETH (`chainId:1 + 0x0`) → `ETH.png`; mainnet RealUnit + Sepolia RealUnit → `REALU.png`; mixed-case address still resolves REALU (production lowercases first); unknown chain/address falls back to `REALU.png` (pinned — changing this would silently render every unknown asset as ETH instead). `getChainImagePath` for mainnet + Sepolia.
• Blockchain: `Blockchain.values` has exactly 2 entries; chainId / name / nativeSymbol for both; `getFromChainId` resolves the known ids and throws `StateError` on unknown; `nativeAsset` shape (address `0x0`, decimals 18).
• xfile_extension: `toBase64DataUri` uses an explicit `mimeType` when present; falls back to extension-based guessing for `.png` / `.jpg` / `.JPEG` (case-insensitive) / `.pdf`; defaults to `application/octet-stream` for unknown extensions; the base64 segment encodes the raw file bytes (verified by round-tripping a UTF-8 payload).

Notes

• `xfile_extension` tests write a real `XFile` against a per-test temp directory so the production code's `readAsBytes` path runs unchanged. No platform-channel plumbing needed.

Test plan

• `flutter analyze` clean
• `flutter test` — 22 / 22 passing locally
• CI green