reth-entrypoint: set NODE_TYPE default to vanilla (OP Reth)

[phrwlk]

Align the NODE_TYPE default with README and docker-compose. Previously, the script defaulted to base, causing standalone runs to unintentionally start Base mode. This change ensures OP Reth (vanilla) is the default unless explicitly overridden, matching documented behavior and compose defaults.

[cb-heimdall]

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[mur-me]

Haha, I'm not the only one who noticed this

[haardikk21]

Base node without flashblocks is the same thing as vanilla Reth. We are removing the NODE_TYPE entirely and "vanilla" mode is the base node with no flashblocks websocket url supplied. Will be done in #634