fix: prevent SageMaker TGI from being marked configured when only Bedrock keys are set

[ibaaaaal]

Summary

Fixes #7268 — Configuring Amazon Bedrock causes Amazon SageMaker TGI to appear as configured.

This PR also fixes a related issue: Amazon Bedrock itself was not appearing in the provider selection dropdown because all its config keys were optional.

Root Cause

Issue 1: SageMaker TGI incorrectly marked as configured

SAGEMAKER_ENDPOINT_NAME was declared as required=false in the provider metadata, while AWS_REGION and AWS_PROFILE were required=true with defaults. When a user configures Bedrock, the shared AWS_REGION/AWS_PROFILE keys get stored globally. The check_provider_configured() logic then sees SageMaker TGI's required keys are satisfied and marks it as configured, even though no SageMaker endpoint was actually set up.

Issue 2: Bedrock not appearing in provider dropdown

Amazon Bedrock had all config keys marked as required=false. The check_provider_configured() function returns false for providers with zero required keys, causing Bedrock to never appear as "configured" in the provider selection dropdown.

Fix

SageMaker TGI fix (sagemaker_tgi.rs):

• Mark SAGEMAKER_ENDPOINT_NAME as required=true — it is already required at runtime
• Mark AWS_REGION and AWS_PROFILE as required=false — consistent with Bedrock

Bedrock fix (bedrock.rs):

• Mark AWS_REGION as required=true with default "us-east-1" — this ensures Bedrock is properly marked as configured when the region is set
• Update test assertion to reflect correct expected behavior

Validation

• cargo fmt --check — pass
• cargo clippy -D warnings — pass
• cargo test --release -p goose — 687 tests pass (4 unrelated failures in prompt_manager are pre-existing)