docs: test mode — concept page, authentication page, test-first quickstart

[riveign]

What

Public docs coverage for test mode, targeting the Jul 15 GA. Follows the Test Mode — Public Docs Alignment doc + the comments on it. Opened as a draft to iterate on — comment freely.

New pages

• payments/merchant/test-mode — the concept page: account-level test/live, switch-by-key (no test flag/param anywhere), wcp_test_ prefix, what test payments do/don't do (no on-chain execution, synthetic test:{paymentId} tx hash, fixed 1-unit amounts), webhooks fire in test mode, go-live path. Includes an environment diagram with test mode in purple (per Rocky's diagram ask + Tomas R.'s purple-as-test-color proposal).
• api-reference/authentication — the previously-missing auth page: Api-Key header, test vs live key semantics, 401/403, key safety.

Edits

• Quickstart reframed test-first: step 1 creates a test key, examples use wcp_test_, synthetic txId in the status example, closing "Go live" section.
• Overview — test-before-live line + Merchant Quickstart / Test Mode cards in Get Started.
• Wallet AI-prompt blurbs (flutter + react-native) — the two contradictory "Test Payment Links" sections now agree (dashboard self-serve); the docs' last "sandbox" mention is gone (canonical term: test mode).
• Nav (docs.json) — both version blocks updated.

Decisions baked in (from the alignment thread)

• Devs use the same API as today — the environment is selected by the key alone.
• Docs explicitly state that responses/webhooks carry no environment field (it's determined by your key).
• The &_t=1 gateway tag is treated as an implementation detail — not documented.

Not in this PR (follow-ups)

• OpenAPI snapshots are stale — they still advertise the dead not_sandbox_api_key enum. They should be regenerated, not hand-edited, so the JSONs are untouched here.
• BX page — Rocky asked whether test mode needs a page on the buyer-experience side; open question, happy to add in a follow-up.
• Dashboard screenshots of test-key provisioning (and purple test-mode UI) once the GA build is final.

mint broken-links passes.

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