okhttp: avoid unnecessary Header allocation for lowercase names

[skrcode]

This is an optional, independently reviewed follow-up to grpc#12907. It does not open a duplicate PR against upstream.

What

ByteString.toAsciiLowercase() leaves an already-lowercase header name unchanged. For that common gRPC path—including :path and :authority—the writer can reuse the immutable input Header in the dynamic table instead of allocating an equivalent object. Mixed-case input still receives a canonical lowercase Header; focused tests lock both behaviors.

Evidence

• JAIPilot instrumented a representative lowercase gRPC header block at 5 → 0 new Header objects per writeHeaders call.
• Its five-run ThreadMXBean harness reported 8,488 → 8,368 allocated bytes/iteration over 200,000 iterations per side. This is allocation evidence, not a wall-clock claim.
• The committed lowercaseHeaderIsReusedInDynamicTable test deterministically asserts the allocation-saving branch by checking that the writer stores the same immutable Header instance.
• Independent Corretto 21 replay of the exact source head: clean :grpc-okhttp:build passed, 359 tests, 0 failures/errors (3 skipped).
• Independent replay of this exact candidate: the same 50-task clean build passed, 361 tests, 0 failures/errors (3 skipped).
• The branch starts directly at exact source head d0272ef78f4e78983cb1f19ac25d1561ec26cc1b.

The original bot draft is skrcode#2. Independent review replaced its intentional reference comparison with value equality because the reference form introduced an Error Prone warning; this version preserves the zero-allocation common path without adding that warning.

Scope and limitations

Two existing files, no public API or wire-format change. The ThreadMXBean harness was intentionally not committed, so the durable proof is the identity assertion, mixed-case regression, and identical repository-native clean builds. No wall-clock speed claim is made.

Both commits include a Generated-by: JAIPilot Cloud trailer for transparency.