refactor(handler): remove Default for EthPrecompiles and EthInstructions

[rakita]

Summary

• Remove impl Default for EthPrecompiles — require explicit EthPrecompiles::new(spec) construction
• Remove impl Default for EthInstructions — require explicit EthInstructions::new_mainnet() construction
• Update all examples to use explicit constructors
• Simplify cheatcode_inspector example by removing unnecessary InstructionProviderT/PrecompileT generic parameters (they only existed to satisfy Default bounds)

Test plan

• cargo check --workspace passes
• cargo clippy --workspace --all-targets --all-features passes clean
• cargo fmt --all applied

[codspeed-hq]

Merging this PR will not alter performance

✅ 176 untouched benchmarks

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_{Comparing rm-default-eth-precompiles-instructions (08e532a) with main (39564c9)}