Allow repeated User-Agent headers to merge instead of raising duplicate-header errors

[Copilot]

Some clients (notably crawlers) send multiple User-Agent header lines. Protocol::HTTP::Headers previously treated user-agent as singleton, causing DuplicateHeaderError and dropped requests when duplicates were present.

• Policy change

  • Updated Headers::POLICY to treat user-agent as Header::Split (mergeable) rather than false (singleton).
  • Duplicate User-Agent lines now coalesce into a single logical header value instead of failing request parsing.

• Spec coverage

  • Added a focused headers spec asserting duplicate user-agent values are merged and serialized consistently with existing split-header behavior.

headers = Protocol::HTTP::Headers.new
headers.merge!([
  ["user-agent", "foobar/2000"],
  ["user-agent", "foobar/2001"],
])

headers["user-agent"]      # => ["foobar/2000", "foobar/2001"]
headers["user-agent"].to_s # => "foobar/2000,foobar/2001"