Fix dark cluster URI rewrite deopt storm consuming 80% CPU

[jcleezer]

Summary

• Adds a skipReEncoding fast path to D2URIRewriter that constructs rewritten URIs directly from raw (already percent-encoded) components via StringBuilder + URI.create, bypassing the expensive character-by-character
  UriComponent._encode loop.
• Opts in from DarkClusterManagerImpl only (new D2URIRewriter(configuredURI, true)). All other callers use the default constructor and are completely unaffected.
• Existing UriBuilder-based path is preserved unchanged behind skipReEncoding=false (the default).

Problem

Production profiling shows DarkClusterFilter.onRestRequest → DarkClusterManagerImpl.rewriteRequest → D2URIRewriter.rewriteURI → UriBuilder.replaceQuery → UriComponent._encode consuming ~80% of total CPU.

The root cause is a JIT deoptimization storm on _encode:

• The method has bimodal branching (StringBuilder sb = null fast path vs slow path) that C2 speculatively compiles for one mode
• Varying query strings across requests invalidate the speculation → uncommon trap → deopt to interpreter
• Running the character-by-character loop interpreted over large query strings is 10-100x slower
• Recompilation uses contaminated profiles → the cycle repeats

The re-encoding is also unnecessary: getRawQuery() returns already percent-encoded strings, so _encode scans every character only to leave them unchanged.

Fix

rewriteURIFromRaw eliminates the problem entirely:

• No character-by-character iteration — StringBuilder.append(String) is a JIT intrinsic (System.arraycopy)
• No bimodal branching — no speculative optimization to get wrong
• Monomorphic call sites — stable JIT targets with no deopt triggers

Test plan

• Added skipReEncoding tests to TestD2URIRewriter (simple rewrite, encoded query params, no query, with fragment)
• Added skipReEncoding tests to TestDarkClusterUrlRewrite (basic, query params, encoded params, parity with default)
• Tests explicitly assert fast path produces identical results to the default UriBuilder path
• ./gradlew :d2:test --tests TestD2URIRewriter :darkcluster:test --tests TestDarkClusterUrlRewrite passes