IDC login fails with REQUEST_BODY_INVALID when profile does not support getUsageLimits

[quanhk711]

Describe the bug
IAM Identity Center (IdC) login fails with 400 REQUEST_BODY_INVALID when the selected CodeWhisperer/Q Developer profile does not support the getUsageLimits API. A valid token is actually issued during the device-code flow, but the auth callback throws on the optional usage check and aborts the entire login.

To Reproduce
Steps to reproduce the behavior:

1. Configure ~/.config/opencode/kiro.json for IdC with a profile ARN whose profile does not support the usage-limits API (e.g. a QDefaultProfile):
   • idc_start_url: https://<org>.awsapps.com/start
   • idc_region: eu-west-1
   • idc_profile_arn: arn:aws:codewhisperer:us-east-1:<acct>:profile/<id>
2. Run opencode auth login.
3. Select provider kiro → login method IAM Identity Center with Profile ARN.
4. Complete the device-code sign-in at the verification URL.
5. See error: Failed to authorize: IDC authorization failed: Status: 400 (ValidationException) [<request-id>]: {"message":"Improperly formed request.","reason":"REQUEST_BODY_INVALID"}.

Expected behavior
Login should succeed. A failure of the optional getUsageLimits call (usage tracking is optional) should never block authentication — it should be skipped gracefully, the same way FEATURE_NOT_SUPPORTED already is. Setting usage_tracking_enabled: false should also skip the usage call during login.

Screenshots
Log from ~/.config/opencode/kiro-logs/plugin.log:
INFO: IDC authorize: resolved defaults {"oidcRegion":"eu-west-1","startUrl":"https://.awsapps.com"}
WARN: fetchUsageLimits failed during auth {"error":"Status: 400 (ValidationException) ...REQUEST_BODY_INVALID"}
ERROR: IDC auth callback failed Error: Status: 400 ...REQUEST_BODY_INVALID
at fetchUsageLimits (dist/plugin/usage.js:47:27)
at async dist/core/auth/idc-auth-method.js:95:39

Direct API test against q.us-east-1.amazonaws.com/getUsageLimits with a valid token + profile ARN (this profile supports none of the param combos):

Attempt (params)	HTTP	Body
AGENTIC_REQUEST + AI_EDITOR	403	FEATURE_NOT_SUPPORTED
AI_EDITOR only	403	FEATURE_NOT_SUPPORTED
CONVERSATION + AI_EDITOR	400	REQUEST_BODY_INVALID
no resourceType	403	FEATURE_NOT_SUPPORTED

Desktop (please complete the following information):

• OS: macOS (darwin, Apple Silicon / arm64)
• Browser: N/A (CLI / OpenCode)
• Version: @zhafron/opencode-kiro-auth@2.0.0; auth method: IAM Identity Center with Profile ARN

Smartphone (please complete the following information):

• Device: N/A
• OS: N/A
• Browser: N/A
• Version: N/A

Additional context
Root cause is in the auth callback in dist/core/auth/idc-auth-method.js (~line 113), which only swallows FEATURE_NOT_SUPPORTED:

if (errMsg.includes('FEATURE_NOT_SUPPORTED')) {
    // skip usage check
} else {
    throw e; // REQUEST_BODY_INVALID lands here and aborts login
}
Suggested fix — treat REQUEST_BODY_INVALID the same as FEATURE_NOT_SUPPORTED:
if (errMsg.includes('FEATURE_NOT_SUPPORTED') || errMsg.includes('REQUEST_BODY_INVALID')) {
    usage = { usedCount: 0, limitCount: 0, email: undefined };
} else {
    throw e;
}
Also consider honoring usage_tracking_enabled: false during login — currently that flag is only checked at runtime (auth-handler / usage-tracker), not in the login path, so fetchUsageLimits is called unconditionally and setting the flag alone does not fix login.
Workaround: patching the condition above lets login complete (email falls back to a placeholder, token is stored and works).