fix(api): box tenant limit rejection responses

[haasonsaas]

Summary

• box the tenant-limit middleware's error response on its cold rejection path
• preserve response status, headers, body, counter semantics, and success-path allocation behavior
• restore the floating Rust 1.98 Clippy gate without suppressing the lint or mixing this prerequisite into fix(api): diagnose empty sterile cell claims #408

Diagnosis

Rust 1.98's clippy::result_large_err now rejects the pre-existing consume(...) -> Result<(), Response> signature in limits.rs. Current main reproduces the failure and no open Sandboxwich PR owns it. The large Axum response is only constructed on database/rate-limit rejection paths, so boxing the error variant is the smallest behavior-preserving fix.

Validation

• cargo +stable fmt --check
• cargo +stable clippy --workspace --all-targets -- -D warnings
• cargo +stable test --workspace
• cargo +1.95.0 fmt --check
• cargo +1.95.0 clippy --workspace --all-targets -- -D warnings
• focused SQLite tenant-limit HTTP contract passed
• PostgreSQL tenant-limit case was skipped locally because SANDBOXWICH_TEST_POSTGRES_URL is not configured

No deployment or production mutation was performed.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.