Improve wallet repair with previews, validation, error handling, and multi-coin support

[QuickMythril]

Summary

• Enforce deterministic private keys for repair, rejecting xpub/tpub on sweep endpoints.
• Surface proper API errors on repair failures instead of returning non-txid strings.
• Add repair preview summaries and gate repair behind recommendations with a force override.
• Extend repair/preview behavior to BTC, DOGE, DGB, and RVN.
• Strengthen unit and API tests to validate txid format and new preview/force behavior.

Issue

Refs: #294

Commit breakdown

Require private key for litecoin repair

• src/main/java/org/qortal/api/resource/CrossChainLitecoinResource.java: validate deterministic private keys for repair so xpub/tpub are rejected.
• src/main/java/org/qortal/crosschain/Bitcoiny.java: add a private-key validator helper used by repair endpoints.

Return API errors for wallet repair failures

• src/main/java/org/qortal/api/resource/CrossChainLitecoinResource.java: map repair failures to API errors so clients get clear error responses.
• src/main/java/org/qortal/crosschain/Bitcoiny.java: throw structured exceptions instead of returning class-name strings.
• src/test/java/org/qortal/test/crosschain/BitcoinyTests.java: align tests with the new error handling.

Assert repair returns valid txid

• src/test/java/org/qortal/test/crosschain/BitcoinyTests.java: verify txid format rather than just non-null responses.

Add litecoin repair preview endpoint

• src/main/java/org/qortal/crosschain/RepairWalletPreview.java: new response model for preview summaries.
• src/main/java/org/qortal/crosschain/Bitcoiny.java: implement old-vs-current scan comparison, missing balance, fee/dust estimate, and recommendation logic.
• src/main/java/org/qortal/api/resource/CrossChainLitecoinResource.java: add /repair/preview endpoint to expose the summary.
• src/test/java/org/qortal/test/crosschain/BitcoinyTests.java: add preview test coverage.

Gate litecoin repair behind recommendation

• src/main/java/org/qortal/api/resource/CrossChainLitecoinResource.java: refuse repair when preview is not recommended unless force=true.

Add repair preview and gated repair for BTC/DOGE/DGB/RVN

• src/main/java/org/qortal/api/resource/CrossChainBitcoinResource.java: add preview endpoint and force-gated repair.
• src/main/java/org/qortal/api/resource/CrossChainDogecoinResource.java: add preview endpoint and force-gated repair.
• src/main/java/org/qortal/api/resource/CrossChainDigibyteResource.java: add preview endpoint and force-gated repair.
• src/main/java/org/qortal/api/resource/CrossChainRavencoinResource.java: add preview endpoint and force-gated repair.

Document force query param in repair endpoints

• src/main/java/org/qortal/api/resource/CrossChainBitcoinResource.java: OpenAPI @Parameter docs for force.
• src/main/java/org/qortal/api/resource/CrossChainLitecoinResource.java: OpenAPI @Parameter docs for force.
• src/main/java/org/qortal/api/resource/CrossChainDogecoinResource.java: OpenAPI @Parameter docs for force.
• src/main/java/org/qortal/api/resource/CrossChainDigibyteResource.java: OpenAPI @Parameter docs for force.
• src/main/java/org/qortal/api/resource/CrossChainRavencoinResource.java: OpenAPI @Parameter docs for force.

Add API tests for repair preview and force

• src/test/java/org/qortal/test/api/CrossChainRepairApiTests.java: new API-level tests for preview summaries and force-gated repair behavior using a dummy provider to avoid network calls.

Fix repair test compilation and exception ctor

• src/main/java/org/qortal/crosschain/ForeignBlockchainException.java: add a (String, Throwable) constructor so repair paths can preserve causes without build errors.
• src/test/java/org/qortal/test/api/CrossChainRepairApiTests.java: rely on the static ApiCommon.assertApiError helper to avoid an invalid instance override and keep tests compiling.