feat(http): add body regex filtering

[Co-Messi]

Problem

HTTP filtering currently supports method, path, prefix, regex, and host matching, but there is no way to filter on request or response body content.

What changed

• added a --body regex flag to the http command
• extended protocol.HttpFilter with body regex support
• matched the regex against the HTTP payload section after `

`

• enabled response-side filtering when a body regex is configured
• added HTTP filter tests for request-body matches, response-body matches, and non-matches

Why this fixes it

This keeps the new behavior inside the existing HTTP filter path, so body matching works alongside the current path/method/host filters instead of introducing a separate filtering stage.

Verification

• GOOS=linux GOARCH=arm64 go test -c ./agent/protocol
• parser sanity check for the new request/response body fixtures using Go's stdlib net/http

I could not run the full Go test suite on this macOS machine because the repo's Linux-only code and BPF toolchain are required for end-to-end test execution.

Fixes #263

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