[codex] Support callbacks, webhooks, and links in HTTP artifacts

[HavenDV]

Summary

• retain callback and webhook request/response schemas in the OpenAPI traversal pipeline
• surface callbacks and response links in generated .http artifacts
• emit a dedicated webhooks.http artifact for root-level OpenAPI webhooks and document the strategy in the README
• add a fixture spec plus focused unit coverage for callbacks, webhooks, and links

Why

#226 pointed out that callbacks, webhooks, and links were effectively invisible to AutoSDK's traversal and HTTP artifact generation. This change keeps the inbound-contract pieces in the pipeline without turning them into normal outbound SDK methods.

Impact

• callback and webhook schemas now participate in model traversal and generation
• autosdk http documents callback and link metadata next to the owning operation
• autosdk http writes a webhooks.http file for inbound webhook contracts

Validation

• dotnet build src/libs/AutoSDK.CLI/AutoSDK.CLI.csproj
• dotnet test src/tests/AutoSDK.UnitTests/AutoSDK.UnitTests.csproj
• dotnet test src/tests/AutoSDK.IntegrationTests.Cli/AutoSDK.IntegrationTests.Cli.csproj --filter "FullyQualifiedName~Http"

Closes #226