Avoid allocations when escaping safe strings

[quinnj]

escapeuri and escapepath currently allocate even when a String needs no escaping. Return that immutable string directly for the built-in policies. If an escape occurs, reuse the checked prefix without repeating its predicate checks. Custom predicates keep their existing byte order and call count.

Warmed Julia 1.12.7 allocations: bucket 96→0 B; a 3,072-byte safe string 6,336→0 B; /bucket/key 112→0 B. Escaped-input allocations stayed unchanged across 432 cases on each tested version. Alternating local timing runs improved long escaped prefixes; short escaped/Unicode cases were roughly 2–3 ns slower. The host had concurrent work, so these are diagnostic timings.

Validation: full suites on Julia 1.6.7 and 1.12.7; 12,630 independent semantic comparisons per version; allocation guards; safe-trim compilation and 10 executable checks. Covers malformed UTF-8, substrings, and stateful custom predicates.

The candidate also passed 155 combined checks with HTTP #1374, CloudBase #48/#49 and CloudStore #71/#72, plus all eight HTTP buffered-transfer allocation guards. These used local mocks/storage emulators; no cloud-throughput claim.

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.87%. Comparing base (a045d2d) to head (3686b7a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   87.05%   87.87%   +0.81%     
==========================================
  Files           5        5              
  Lines         394      404      +10     
==========================================
+ Hits          343      355      +12     
+ Misses         51       49       -2     

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