Fix premine reward height validation

[NateIsern]

Motivation

• Correct an off-by-one in reward validation that shifted the 5,000,000 FAIR premine from block 1 to block 2, which could allow an early block producer to claim the premine or cause consensus splits.

Description

• Compute the expected mint using the current block height in ConnectBlock by replacing GetBlockValue(pindex->pprev->nHeight) with GetBlockValue(pindex->nHeight) in src/main.cpp so the premine at height 1 is validated at height 1.
• Extend the subsidy unit test in src/test/main_tests.cpp to assert that block 2 returns the regular 10 FAIR subsidy (and leave existing checks for block 0 and block 1 intact).

Testing

• Ran git diff --check, which reported no issues (success).
• Ran ./autogen.sh in this environment which failed due to missing aclocal, so a full build or unit test execution was not possible here (failure).

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Codex Task