Make prod fork skips explicit

[TUPM96]

Closes #213.

Summary

• fail fast with explicit errors when RPC_URL_BASE_FORK or FORK_BLOCK_BASE is missing
• replace silent zero-address returns with visible vm.skip locally and hard failures when REQUIRE_PROD_DEPLOYED=1
• apply the same visible/fail-fast behavior to inline multi-feed _existsOnFork guards
• set REQUIRE_PROD_DEPLOYED=1 in the Rainix CI job so production fork coverage cannot pass vacuously

Validation

• forge fmt --check test/src/concrete/ProdFork.t.sol
• forge build
• forge test --match-contract ProdForkTest --match-test testProdWtcoinOracleLatestAnswer -vv with fork env vars removed: fails as expected with MissingForkRpc()
• RPC_URL_BASE_FORK=https://mainnet.base.org FORK_BLOCK_BASE=38996123 forge test --no-match-contract ProdForkTest: 121 passed
• RPC_URL_BASE_FORK=https://mainnet.base.org FORK_BLOCK_BASE=46456804 REQUIRE_PROD_DEPLOYED=1 forge test --match-contract ProdForkTest --match-test testProdWtcoin(OracleDecimals|MultiOracleDecimals|MorphoConfig|PassthroughDecimals) -vv: 4 passed

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Configuration used: defaults

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Run ID: ce6b33e2-2a2d-4b91-b8ff-8d4fc160aab6

📥 Commits

Reviewing files that changed from the base of the PR and between 3a24b5d and fb18454.

📒 Files selected for processing (2)

• .github/workflows/rainix.yaml
• test/src/concrete/ProdFork.t.sol

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