fix(http): preserve Content-Length header for streaming request bodies

[robobun]

Summary

• Fixes user-provided Content-Length headers being stripped when the request body is a stream (async generator or ReadableStream)
• Bun now uses the user-provided Content-Length instead of replacing it with Transfer-Encoding: chunked

Test plan

• Added regression tests in test/regression/issue/18854.test.ts:
  • Test that Content-Length is preserved with async generator body
  • Test that Content-Length is preserved with ReadableStream body
  • Test that chunked encoding is still used when no Content-Length is provided

Fixes #18854

🤖 Generated with Claude Code

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[robobun]

^{Updated 2:50 AM PT - Jan 27th, 2026}

❌ Your commit ce7394ea has 6 failures in Build #35936 (All Failures):

• test/bake/dev/react-response.test.ts - 2 failing on 🍎 14 x64
• test/js/bun/perf/static-initializers.test.ts - 1 failing on 🍎 14 x64
• test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🍎 14 aarch64
• test/js/node/worker_threads/worker_threads.test.ts - pid 4311 segmentation fault on 🐧 13 x64
• test/bundler/compile-windows-metadata.test.ts - 1 failing on 🪟 2019 x64
• test/bundler/compile-windows-metadata.test.ts - 1 failing on 🪟 2019 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🪟 2019 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🪟 2019 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 25.04 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 13 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 3.22 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 3.22 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 25.04 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 13 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 3.22 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🐧 25.04 x64-baseline
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 14 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 14 x64
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 13 aarch64
• test/js/sql/tls-sql.test.ts - 9 failing on 🍎 13 x64

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🧪   To try this PR locally:

bunx bun-pr 26490

That installs a local version of the PR into your bun-26490 executable, so you can run:

bun-26490 --bun