[codex] fix Buffer deprecations

[bobzhang]

Summary

Replace the deprecated @buffer.new() constructor with Buffer().

This PR is intentionally scoped to Buffer deprecations only, split out from #392.

Changed files

• src/fs/tmpdir.mbt
• src/http/send.mbt
• examples/tcp_server_benchmark/main.mbt

Validation

• moon check --deny-warn
• moon check --target js --deny-warn
• moon check examples/tcp_server_benchmark --deny-warn
• git diff --check

Known follow-up

• moon test src/fs src/http still hits the existing cold-build src/fs/lock_test.mbt race from [codex] fix MoonBit deprecations #392; I left that out of this PR to keep this split focused on Buffer only.

[coveralls]

Coverage Report for CI Build 357

Coverage increased (+0.03%) to 79.379%

Details

• Coverage increased (+0.03%) from the base build.
• Patch coverage: 1 of 1 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4190
Covered Lines:	3326
Line Coverage:	79.38%
Coverage Strength:	80304.02 hits per line

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💛 - Coveralls

[bobzhang]

Split/CI status for this Buffer-only PR:

• Scope is intentionally limited to @buffer.new() -> Buffer() replacements: 3 one-line changes.
• Local validation passed:
  • moon check --deny-warn
  • moon check --target js --deny-warn
  • moon check examples/tcp_server_benchmark --deny-warn
  • git diff --check
• GitHub Actions: all build/test/coverage/sanitizer jobs passed. The remaining red checks are repo-wide toolchain drift that I kept out of this PR to preserve the split:
  • moon-format wants broad formatter updates plus moon.mod.json -> moon.mod migration.
  • moon-info wants generated .mbti signature syntax updates.

I will handle those as a separate split instead of folding them into this Buffer-only PR.