refactor: resolve SonarCloud quick-win code smells

[nyg]

Resolves the open SonarCloud code smells reported on nyg_kraken-api-java, fixing the mechanical ones and suppressing the ones that reflect deliberate API design.

Tests

• S5778 (46): builder validation tests now build the Lombok builder before assertThatThrownBy(builder::build), so the lambda holds a single call that can throw.
• S1612 (18): KrakenAPI credential tests use method references such as unit::accountBalance instead of lambdas.
• S5838: LedgerInfoEndpointTest asserts with containsEntry.
• S1130: removed an unused throws Exception from TradeVolumeEndpointTest.

Library

• S1192: DefaultKrakenRestRequester uses a CONTENT_TYPE constant for the header name.
• S7467: DepositLimitDeserializer and LimitValueDeserializer catch NumberFormatException with an unnamed variable.
• S6353: JsonPostParams matches the nonce with \d instead of [0-9], which is equivalent since Java's \d is ASCII-only by default.
• S1488: WalletAccountsParams.params() returns the map directly.
• S1168: CreateFundingWithdrawalParams.fee() returns an empty map instead of null when no fee option is set, and body() only adds fee when it is not empty. A set fee always contains quoted_fee or current_fee, so the encoded body is unchanged.

Suppressed

• S1452 on PostParams.params(): subclasses override it returning either Map<String, String> or Map<String, Object>, which the wildcard allows; removing it would break subclasses of the published API.
• S6539 on KrakenAPI: it is the single entry point exposing every endpoint by design.
• S1168 on FundingBetaParams.body(): null is the documented way for an endpoint to send no body, distinct from an empty JSON object.

Remaining

• S1135 (4): TODO comments that need a design decision rather than a mechanical fix.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
88.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.