feat: support json v2 ephemeral cards

[alick-liming]

Summary

• Allow ephemeral message requests to send JSON 2.0 through CardV2 json.RawMessage, while preserving Card *MessageContentCard for V1 callers.
• Serialize exactly one card object, reject conflicting V1/V2 inputs, and omit an unset card. Cover all four cases with tests.
• Repair outdated test fixtures: initialize method options in all seven request-parser cases and remove the obsolete Range field from the whole-file download test. Existing assertions remain intact.

Validation

• go test . -run '^Test_Request$' -count=1 — passed; reproduced the nil method-option panic before the fixture fix.
• go test . -run '^TestSendEphemeralMessageReq' -count=1 — passed.
• go test ./... -run '^$' -count=1 — all test packages compile.
• go vet ./... — passed.
• go build ./... — passed.
• go mod verify — passed.
• git diff --check — passed.
• go test ./... -count=1 -timeout=60s — the root, card, internal, and larkext packages passed. The test package reached the explicit 60-second limit in Test_CoreHR_Sample_Failed/response_is_failed while requesting a tenant token from Feishu. This is not a full-suite pass; external integration validation remains outstanding.

Scope / Notes

• Production changes are limited to ephemeral card input and serialization. The request-parser and download fixes only change tests.
• The API field is a manually maintained fork extension. This repository has no corresponding API generator/spec or local regeneration entry point; preserve CardV2 when importing regenerated SDK code. This is a maintenance note, not a demonstrated failure in the current build.
• The existing scientific-notation parser fix and version constant update in the fork remain included.