feat(coderd_ai_provider): expose server-generated Bedrock external_id

[ethanndickson]

coder/coder#26869 added a server-generated STS external ID to Bedrock AI provider settings: when role_arn is set, the server generates an ID and sends it on every AssumeRole call, so operators can require it in the role's trust policy via an sts:ExternalId condition (the standard confused-deputy mitigation). This exposes it on coderd_ai_provider as a read-only settings.bedrock.external_id attribute, so the trust policy can reference it in the same Terraform config that manages the provider.

Design notes:

• The value is never sent outbound (the server rejects client-supplied values on create and non-matching values on update), only read back into state; empty responses from servers predating #26869 map to null, so the attribute works against both old and new coderd with no version gating.
• A custom plan modifier preserves a stored ID (the server carries it forward even if role_arn is later cleared), plans unknown while role_arn is set or unknown, and pins null otherwise. UseStateForUnknown would be wrong here per the "computed value derived from mutable config" anti-pattern: the empty-to-set role_arn transition would produce "inconsistent result after apply".

Builds on #381 (SDK bump). Follows up #372 (role_arn).

Closes AIGOV-504

[ethanndickson]

• feat(coderd_ai_provider): expose server-generated Bedrock external_id #382 👈 (View in Graphite)
• chore: bump coder/coder SDK and adapt to chat model config changes #381
• main

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[chatgpt-codex-connector]

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[linear-code]

AIGOV-504

[ethanndickson]

@codex review

re: Preserve null external IDs from unsupported servers

Added a test case to prove this is a non-issue

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

Reviewed commit: d10c7c7e63

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