fix: reserve event cpi discriminator in dispatch

[abhinavgautam01]

Fixes #4272

Summary

• Dispatches the reserved event-CPI tag before user instruction discriminators.
• Prevents a custom short discriminator like [0xe4] from shadowing the full event-CPI tag.
• Adds a regression test showing that the overlapping short custom discriminator still dispatches normally when sent by itself.

Testing

• rustfmt --check lang/syn/src/codegen/program/dispatch.rs tests/events/programs/events/src/lib.rs
• git diff --check
• cargo test -p anchor-syn
• cargo test --manifest-path tests/events/programs/events/Cargo.toml
• cargo test -p tests-v2 event_cpi
• PATH="$HOME/.local/bin:$HOME/.nvm/versions/node/v20.19.0/bin:$PATH" ../../target/debug/anchor test --skip-lint from tests/events

Full tests/events result: 5 passing