feat(go/http): add AVM paywall handler

[wnjoon]

Description

• Add a built-in Go HTTP paywall handler for AVM / Algorand networks.
• Wire algorand:* payment requirements to the generated AVMPaywallTemplate.
• Include AVM coverage in the default paywall provider and paywall selection tests.

Changes

• Added AVMPaywallHandler in go/http/paywall.go.
• Updated DefaultPaywallProvider() to include EVM, SVM, and AVM handlers.
• Updated legacy template selection so algorand:* networks render the AVM paywall template.
• Added tests for AVM handler support and built-in AVM paywall fallback.
• Added a Go changelog fragment.

Tests

cd go
go test ./http -run 'Paywall|AVM' -count=1
go test ./...

Checklist

• [v] I have formatted and linted my code
• [v] All new and existing tests pass
• [v] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• [v] I added a changelog fragment for user-facing changes (docs-only changes can skip)

Related: #2160 adds chain-aware faucet URL support across paywall templates. This PR complements it by wiring the existing AVM template into the Go HTTP paywall provider and network selector.

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