[AIG-1532] Add a gateway-level setting to force BYOK - #33229

Fix the following review findings in PR #33229 (https://github.com/cloudflare/cloudflare-docs/pull/33229).

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## Code Review

### Warnings (3)

#### CR-f8e1e03c9741 · Factually contradictory billing claim
- **File:** `src/content/docs/ai-gateway/configuration/manage-gateway.mdx` line 87
- **Issue:** The new sentence states Workers AI requests "use standard Workers AI account billing, not AI Gateway Unified Billing." The "Configure Workers AI billing" section on the same page (above) instructs users to select **Unified billing** so that "Workers AI requests routed through this gateway will deduct from your AI Gateway credit balance" — so Workers AI requests *can* use AI Gateway Unified Billing depending on the gateway's Workers AI billing setting. As written, the new sentence is incorrect for any gateway configured with Unified billing and contradicts the page's own instructions.
- **Fix:** Clarify that the statement refers to the default billing mode or to the Unified Billing fallback that this setting disables, e.g. "They use the gateway's configured Workers AI billing mode (standard or unified) — this setting only disables the Unified Billing fallback for third-party provider requests."

#### CR-812563c4074b · Contradicts page's own billing docs
- **File:** `src/content/docs/ai-gateway/features/unified-billing.mdx` line 97
- **Issue:** The added text claims Workers AI requests "use standard Workers AI account billing, not AI Gateway Unified Billing," but the same page's "Use Unified Billing" section states that when the gateway's Workers AI Billing setting is set to "Unified billing," Workers AI requests routed through that gateway deduct from the prepaid credit balance. The two statements conflict for users with that configuration.
- **Fix:** Reword to avoid the billing claim, e.g. clarify that this setting does not block Workers AI requests and that Workers AI billing follows the gateway's Workers AI Billing configuration (which can be Unified billing), or otherwise reconcile the statement with the earlier section.

#### CR-6a4c326489e8 · Inaccurate categorical claim contradicts same page
- **File:** `src/content/docs/ai-gateway/configuration/manage-gateway.mdx` line 87
- **Issue:** The added line states Workers AI requests "use standard Workers AI account billing, not AI Gateway Unified Billing" as an unconditional fact, but the same page's "Configure Workers AI billing" section documents a gateway setting that switches Workers AI requests to Unified billing ("Workers AI requests routed through this gateway will deduct from your AI Gateway credit balance in real time"), and unified-billing.mdx lists "To use credits for Workers AI, set your gateway's Workers AI Billing setting to Unified billing" as a prerequisite. The statement is only true for the default (Standard billing) configuration.
- **Fix:** Qualify the claim, e.g. "By default they use standard Workers AI account billing; if the gateway's Workers AI Billing setting is set to Unified billing, they use AI Gateway Unified Billing instead." This setting itself does not change Workers AI billing.