docs: describe idempotent POST charge external IDs

[EfeDurmaz16]

Summary

Documents how to use ChargeParams.ExternalID for idempotent paid POST routes and agent tool calls.

The Go SDK already includes ExternalID in the charge request and echoes it back in the Payment-Receipt. This README section shows how to pass an existing Idempotency-Key header through as the stable operation ID so retries can be distinguished from different paid operations.

Changes

• Add an Idempotent POST Charges section to the README
• Show Idempotency-Key mapped into ChargeParams.ExternalID
• Clarify that retries should reuse the same value and different operations should not share one value

Verification

• git diff --check
• go test ./...

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Clarify that ExternalID does not enforce idempotency

For paid POST retries, ExternalID is only serialized into the challenge/receipt; server.Mpp.Charge does not persist or reject previously seen IDs, so a retry after the paid operation succeeds but the response is lost can still run the POST side effect again unless the application keeps its own idempotency record keyed by Idempotency-Key. This wording makes the new section sound like passing the header through is enough to make retries safe, so it should explicitly mention the required app-side idempotency/cache check before performing the operation.

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[EfeDurmaz16]

Addressed the ExternalID/idempotency clarification in commit d7dce81. The README now states that ExternalID is receipt metadata only and that applications still need their own idempotency record/cache before running paid side effects. Verified with git diff --check and go test ./....