test: Add stubs for SSO login to IAM domain testing

[ziwikiwi]

Problem

** TEST PR DO NOT MERGE **
Tested scoped-down admin credentials to see if we can get to use project credentials for IAM domain activities for an SSO user. There are a couple of places where we need to switch to use the domain type (IAM) instead of the login type, which is set to an SSO user.

Tested by changing the admin project role to a role with just the scoped-down admin credentials, and changing SSORedeemToken to only vend that role's credentials.

Solution

For the credential routing fixes below

Credential Routing Fixes (flagged with smusIamDomainSsoTest)

These places were using DER/domain credentials but need project credentials for IAM domain SSO login. They may need to be flagged for this to work.

Dev Settings Added (settings.ts)

Three new aws.dev.* settings:

Setting	Purpose
smusStubDerCredentials	Bypass /sso/redeem-token, inject hardcoded credentials as DER
smusStubSsoIssuerUrl	Bypass SSO browser login, create fake SSO connection
smusIamDomainSsoTest	Use project credentials for getToolingEnvironment and connection credentials (IAM domain type flag

File	What Changed
sageMakerUnifiedStudioProjectNode.ts	getToolingEnvironment → project creds
sageMakerUnifiedStudioSpacesParentNode.ts	getToolingEnvironment (for SageMaker domain ID) → project creds
sageMakerUnifiedStudioConnectionParentNode.ts	fetchConnections (Data Warehouse/Processing) → project creds
connectionCredentialsProvider.ts	Connection credentials → project creds
smusAuthenticationProvider.ts	getToolingEnvironment in getProjectAccountId → project creds
datazoneClient.ts	getToolingBlueprintName returns ToolingLite when flag set

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[amazon-inspector-ohio]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[github-actions]

• This pull request modifies code in src/* but no tests were added/updated.
  • Confirm whether tests should be added or ensure the PR description explains why tests are not required.

[amazon-inspector-ohio]

✅ I finished the code review, and didn't find any security or code quality issues.