perf: reduce auth header allocations and validate before async

[KENILSHAHH]

Summary

• Replace to_string().parse() with HeaderValue::from() for integer header values (timestamps, nonces), eliminating throwaway heap allocations on every authenticated
  request
• Format UUID API keys into stack-allocated [u8; 36] buffers instead of heap-allocating via to_string()
• Move size validation (missing, scale, zero/negative) above fee_rate_bps() and tick_size() async calls in the limit order builder so invalid orders fail immediately
  without network round-trips

Test plan

• All 527 existing tests pass with --all-features
• Auth header tests verify identical output values
• Order builder tests confirm validation error messages unchanged

---

Note

Medium Risk
Touches request authentication header construction and limit-order validation ordering; while behavior should be equivalent, mistakes could break authenticated calls or change validation timing in a trading-critical path.

Overview
Reduces per-request allocations in auth header generation by constructing numeric headers via HeaderValue::from(...) and formatting UUID API keys into a stack buffer (instead of to_string().parse()) across L1/L2 and Builder local auth flows.

Reorders limit order validation in OrderBuilder<Limit>::build so size checks (missing/scale/non-positive) happen before async fee_rate_bps/tick_size calls, causing invalid orders to fail immediately without network round-trips.

^{Written by Cursor Bugbot for commit 868c3a8. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.56%. Comparing base (a5dc726) to head (868c3a8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
+ Coverage   85.54%   85.56%   +0.02%     
==========================================
  Files          32       32              
  Lines        5167     5175       +8     
==========================================
+ Hits         4420     4428       +8     
  Misses        747      747              

Flag	Coverage Δ
rust	85.56% <100.00%> (+0.02%)	⬆️

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[KENILSHAHH]

Would love the feedback here @chaz-polymarket