Improve HTTP request and response correctness

[ryanfowler]

Summary:

• validate request framing headers before connecting, including body-length consistency and transport-representable framing
• retry additional transient HTTP statuses while avoiding retries for DNS name-not-found failures
• use decoded response length for download progress so compressed downloads do not exceed 100 percent
• document the updated header and retry behavior

Testing:

• GOCACHE=/tmp/fetch-gocache go test -parallel 8 ./...
• XDG_CACHE_HOME=/tmp/fetch-xdg-cache GOCACHE=/tmp/fetch-gocache staticcheck ./...
• GOCACHE=/tmp/fetch-gocache CGO_ENABLED=0 go build -o /tmp/fetch-pr-build .
• GOCACHE=/tmp/fetch-gocache go mod verify

Review:

• completed three independent subagent review rounds
• addressed all actionable feedback, including wire-level framing tests