docs: document Bedrock mantle protocol - #27296

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AIGOV-518

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Updates Needed

• docs/ai-coder/ai-gateway/providers.md - Step 4 of Obtaining static Bedrock credentials still tells users to enter "the region (or base URL) and model identifiers". This PR replaces the separate region/base-URL config with a single endpoint field and states that Mantle providers don't set model identifiers (the client chooses the model per request). Update this step to use the new endpoint terminology and clarify that model identifiers are InvokeModel-only.
• docs/ai-coder/ai-gateway/providers.md - The IAM guidance (the default-credential-chain bullet and step 1 of Assuming an IAM role) documents only bedrock:InvokeModel and bedrock:InvokeModelWithResponseStream. Those are InvokeModel-protocol actions. Mantle applies AWS SigV4 signing against the bedrock-mantle service rather than calling InvokeModel, so its required IAM permissions differ. Document the permissions a Mantle provider needs, or scope the listed permissions to InvokeModel so Mantle users aren't misled.

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@johnstcn

Maybe break it out into separate sections for InvokeModel versus Mantle?

Done.