fix(v2): support explicit bumps with init_if_needed

[abhinavgautam01]

Fixes #2996

Summary

• Allows #[account(init_if_needed, ..., bump = <expr>)] in the v2 derive.
• Keeps #[account(init, ..., bump = <expr>)] rejected so plain init still uses the canonical bump.
• Runs explicit-bump PDA verification only on the existing-account branch of init_if_needed.
• Preserves canonical bump behavior on the create branch, so handlers can store ctx.bumps.<account> for later reuse.

Testing

• rustfmt --check lang-v2/derive/src/parse.rs tests-v2/programs/constraints/src/lib.rs tests-v2/tests/constraints.rs
• cargo test -p anchor-derive-accounts-v2 explicit_bump
• cargo test -p tests-v2 init_if_needed
• git diff --check