fix(http): preserve multiple custom route cookies

[pacocartones]

Summary

• preserve every Set-Cookie value returned by a custom Hono route
• avoid collapsing cookies through repeated Node setHeader calls
• add a regression covering two cookies with distinct attributes

Hono keeps multiple cookies separately, but iterating its headers and calling res.setHeader("set-cookie", value) overwrites the earlier value. This change skips that header during the generic copy and assigns Hono's complete getSetCookie() array once.

Validation

• regression receives only the second cookie on the parent commit and both cookies with this change
• 23 route tests pass
• full lint, type checking and JSR publish dry-run pass

[github-actions]

🎉 This PR is included in version 4.20.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release
• JSR.io

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