W3C Baggage propagation: use percent encoding per spec

[rxchitrx]

fixes #5566

• Replace quote_plus/unquote_plus with quote(safe="")/unquote.
• Spaces encode as %20 and literal + characters are preserved.
• Add regression tests.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rxchitrx / name: Rachit Mittal (008f871)
• ✅ login: rxchitrx / name: rxchitrx (61aaf12)

[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-17T06:27:51.241465Z	61aaf12	PR opened

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