fix(payments): self-heal empty aws-targets.json before T02 deploy

[fahadfa-aws]

Issue

The @aws/agentcore CLI scaffolds <project>/agentcore/aws-targets.json as an empty list. agentcore deploy then exits with Target "default" not found in aws-targets.json before the CDK synth runs, and the user sees a confusing error with no clear next step.

Live-reproduced today on both 0.14.0 and 0.19.0 (current latest). The scaffold writes [] regardless of CLI version, so a fresh user following Tutorial 02 cannot deploy without intervention.

A version pin in the README would not help here — the behavior is consistent across the versions a fresh user is likely to install. This PR uses a property-based check instead: if the file exists but doesn't contain a default target, append one using the account/region the script has already resolved.

Changes

deploy_payment_agent.py: between scaffolding (agentcore create) and deploy (agentcore deploy), check aws-targets.json and append a default entry if missing. Existing entries are preserved. Malformed JSON, unexpected shapes (top-level dict, garbage list entries), and missing files are all left alone so the user sees the underlying CLI error rather than a silent overwrite.

README.md: troubleshooting entry covering the symptom and a one-line manual fix for users running agentcore deploy directly without the script.

Verification

Static — tested the probe in isolation against 8 input shapes:

Input shape	Action
[] (the actual scaffold output)	Append default
File missing	Pass through
[{name: default, ...}]	No-op
Malformed JSON	Pass through
Whitespace-only	Pass through (treated as malformed)
[{name: prod, ...}]	Append default, preserve prod
Garbage list [1, 2, "x"]	Pass through
Top-level dict {...} instead of list	Pass through

All 8 pass. The inserted code in deploy_payment_agent.py is byte-equivalent to the standalone-tested function.

Live AWS — ran end-to-end against us-west-2 on both CLI versions:

CLI version	Scaffold output	Without probe	With probe
0.14.0	[]	Fails: Target "default" not found in aws-targets.json	CFN stack AgentCore-PaymentAgent-default reaches CREATE_COMPLETE
0.19.0	[]	Same failure	Same success

Both stacks were torn down after validation. Variables account_id (line 51) and REGION (line 52) are already in scope at the insertion point. json and os are already imported. The probe is positioned after the existing if not os.path.exists(project_dir) skip so the directory is guaranteed to exist.

[fahadfa-aws]

@mvangara10 — flagging this for your review when you have a moment. Tagged across the full set of payments-tutorial fixes I've been pushing today; happy to walk through any of them. Audit logs and test evidence are referenced in the PR description.

[fahadfa-aws]

Superseded by #1738 (consolidated PR)