fix: heartbeat limit in mvcgen due to withDefault rfl

[sgraf812]

This PR fixes a test case reported by Alexander Bentkamp that runs into a heartbeat limit due to daring use of withDefault rfl in mvcgen.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase bb8d8da1afa0f67db2eb10a34f323d72abdb0a8e --onto c595413fccaa47ebdc399d9b817c17425687fc26. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-25 18:08:48)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase b3b4867d6c65a2342068521d8e1b7922f4bde156 --onto c595413fccaa47ebdc399d9b817c17425687fc26. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-26 16:32:35)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase bb8d8da1afa0f67db2eb10a34f323d72abdb0a8e --onto 8038a8b8904f89ad9542c8eda11379f8f006eab1. You can force reference manual CI using the force-manual-ci label. (2026-02-25 18:08:49)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase b3b4867d6c65a2342068521d8e1b7922f4bde156 --onto 8038a8b8904f89ad9542c8eda11379f8f006eab1. You can force reference manual CI using the force-manual-ci label. (2026-02-26 16:32:38)