feat: add API response text override option

[checkly-rca-bot-angie]

Summary

• Add treatResponseBodyAsText to API check request constructs.
• Emit the option in MaC codegen when set to true.
• Add a synthesis fixture/test that verifies the option appears in generated API check payloads.

Validation

• Passed: git diff --check.
• Passed: CLI pre-commit/commit completed for this branch.
• Blocked locally: COREPACK_HOME=/home/node/.cache/corepack pnpm exec vitest packages/cli/src/constructs/__tests__/api-check.spec.ts --pool forks --poolOptions.forks.singleFork could not start tests because the active pnpm trust policy rejects the existing lockfile entry undici-types@6.21.0 (ERR_PNPM_TRUST_DOWNGRADE).

Notes

• Companion platform PR: https://github.com/checkly/monorepo/pull/2798
• @braxtonj requested for support/customer context; an engineering owner should still do the main review.

[braxtonj]

Slack context: https://checklyhq.slack.com/archives/C024ZAJ938S/p1783116243756079

Palo Alto req on Datadog migration