fix(lang): Include system program AccountInfo in CPI invoke calls

[awneesht]

Summary

Fixes #4132

• Root cause: Anchor's codegen for realloc and init constraints used system program wrapper functions (transfer, create_account, allocate, assign) that called invoke_signed without including the system program's AccountInfo. This works at CPI depth 1 (runtime resolves from transaction accounts) but fails at depth 2+ with "sum of account balances before and after instruction do not match".

• Fix: Replace all wrapper calls in constraints.rs and idl.rs with direct invoke/invoke_signed that include system_program.to_account_info() in the account infos array. Uses invoke (not invoke_signed) for transfer operations where the payer is already a Signer, avoiding unnecessary signer seed lookups. This also eliminates heap allocations from CpiContext/struct construction.

• Files changed:

  • lang/syn/src/codegen/accounts/constraints.rs — Fixed generate_constraint_realloc() and generate_create_account() (covers realloc, init, init_if_needed for all account types)
  • lang/syn/src/codegen/program/idl.rs — Fixed __idl_resize_account
  • tests/realloc-cpi-bug/ — New reproduction test with caller/callee CPI pattern

Test plan

• New tests/realloc-cpi-bug/ — 5/5 passing (direct realloc, CPI realloc grow, CPI shrink, CPI grow-after-shrink)
• Existing tests/realloc/ — 5/5 passing (no regressions)
• Existing tests/cpi-returns/ — 11/11 passing (verifies init codegen path)
• cargo test -p anchor-syn — passes

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[awneesht]

@jamie-osec Would you be able to review this PR when you get a chance? Thanks!