Fail mnemonic generation when entropy is unavailable

[chen-yijiang]

Summary

• make random mnemonic generation throwing instead of trapping or silently continuing
• reject invalid entropy strengths with MnemonicError.invalidStrength
• check SecRandomCopyBytes and fail with MnemonicError.entropyUnavailable unless it returns errSecSuccess
• propagate mnemonic-generation failures through both throwing KeyPair(phrase:) initializers
• add injectable entropy generation for deterministic failure-path coverage
• repair stale API client and transaction test fixtures
• update GitHub Actions dependencies so fork coverage upload works

This is the dependency-side fix for iopay/iopay-ios#1909.

Verification

• swift test --filter "SolanaSwiftUnitTests" --enable-code-coverage: 133 tests passed
• regression coverage covers invalid strength, entropy-source failure, unexpected entropy length, and successful deterministic entropy
• GitHub unit-test workflow passed on commit 0e422434036bef4d85e37b4d6bcc21092bcc2039
• git diff --check

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[chen-yijiang]

Ready for maintainer merge: commit 0e422434036bef4d85e37b4d6bcc21092bcc2039 is CLEAN and the unit-test workflow passed (133/133 tests locally, including entropy failure regressions). The current GitHub identity does not have MergePullRequest permission on this repository. Please squash-merge this PR; ioPay PR #1952 will then be switched from the temporary personal fork pin to the resulting canonical iotexproject/solanaswift commit.