Alidate supported modes

[cryptoryda]

Validate Tempo charge request modes

Summary

• validate supported modes during NormalizeChargeRequest
• apply the same validation while parsing generic request maps
• add regression coverage for invalid local and wire-format values

Why

Unsupported modes previously produced challenges that advertised no usable
credential flow. Rejecting them at request boundaries catches configuration and
wire-format errors before an unsatisfiable challenge is issued.

Testing

go test ./pkg/tempo
go test ./...
go vet ./...

[brendanjryan]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Remove the duplicate validator that fails the new test

For every invalid SupportedModes value, this earlier call returns tempo: unsupported mode ..., so execution never reaches validateChargeModes, while the newly added TestChargeRequestRejectsUnsupportedModes requires the exact error tempo: unsupported charge mode .... Consequently the package test suite fails consistently; use a single validator or align the error contract.

AGENTS.md reference: AGENTS.md:L130-L134

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[brendanjryan]

@cryptoryda -- plz fix conflict 🙇