Return Header from Sv2Frame::get_header

[DeepCogNeural]

Summary

• Change Sv2Frame::get_header() to return Header directly instead of Option<Header>.
• Update related tests and examples to remove impossible None handling.

Fixes #2112.
companion stratum-mining/sv2-apps#584

Verification

• cargo fmt --all --check
• cargo test -p framing_sv2
• cargo test -p codec_sv2
• cargo check --workspace --all-targets
• git diff --check

Companion verification:

• cargo fmt --all --check in integration-tests, miner-apps, pool-apps, and stratum-apps
• cargo check --manifest-path integration-tests/Cargo.toml with stratum-core patched to this PR checkout