chore(go/http): remove unused extractPayment/createHTTPResponse

[feldmannn]

Description

go/http/server.go carries two unexported methods on *x402HTTPResourceServer flagged with //nolint:unused and comments claiming they are "kept for API compatibility":

• extractPayment(adapter HTTPAdapter) *x402.PaymentPayload (line 862)
• createHTTPResponse(paymentRequired x402.PaymentRequired, ...) (*HTTPResponseInstructions, error) (line 939)

Both claims are wrong, and the methods are pure dead duplication of the V2 equivalents:

1. Not part of the public API. Both names start with a lowercase letter; they are unexported. Nothing outside the package can call them.

2. The "V1 to V2 conversion" inside them is illusory. go/types.go:65-74 declares the bare names x402.PaymentPayload, x402.PaymentRequired, x402.PaymentRequirements, and x402.ResourceInfo as Go type aliases of the V2 types:

   type (
       PaymentRequirements = types.PaymentRequirements
       PaymentPayload      = types.PaymentPayload
       PaymentRequired     = types.PaymentRequired
       ResourceInfo        = types.ResourceInfo
   )

   So extractPayment takes a V2 struct, re-wraps it field-by-field into the identical struct, and zeros Accepted with // TODO: Convert. createHTTPResponse does the same shape of work and zeros Resource with // TODO: convert. There is no V1 type involved on either side of the "conversion." The TODOs document field-dropping that is unfixable as written: you would either have to re-introduce V1 types that nothing in this code path uses, or write a no-op identity transform, which is what the methods already are once you remove the field-zeroing.

3. Zero callers in Go. Repo-wide grep finds no .extractPayment( or .createHTTPResponse( call sites in any Go file, and no string-literal references that might be reached via reflection.

This PR deletes both methods and their //nolint:unused suppressions. The V2 methods (extractPaymentV2, createHTTPResponseV2) remain and are unaffected.

Disclosure: this change was prepared with the help of an AI coding assistant. The deletion was independently verified by grep, by reading the type-alias declarations, and by running the full Go test and lint suite locally before commit.

Tests

Local verification with Go 1.23.4 on Windows, against main:

• go build ./...: exit 0
• go vet ./...: exit 0
• go test -cover ./...: exit 0. All packages report ok. The go/http package coverage is 78.8%.
• golangci-lint run --config .golangci.yml ./http/... (v2.x): 0 issues
• gofmt -d on the changed file's git blob: 0 lines (gofmt-clean as committed; CI's make fmt will not produce a diff)
• Caller search: 0 matches for .extractPayment(, .createHTTPResponse(, "extractPayment", "createHTTPResponse" anywhere in go/
• No //go:generate directives, no reflect.MethodByName usage, no build tags referencing the deleted symbols

-race was not exercised locally (no CGO toolchain on this host); the repo's CI race build will catch any (unlikely) race introduced by a pure deletion.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (skipped: deletion is purely internal/unexported, no Go API change)

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