fix(cli): guide Stage users to a working pattern in stack-selection error

[lemon0333]

Fixes #1451

When an app has more than one stack and no selector is given, cdk deploy prints:

Since this app includes more than a single stack, specify which stacks to use (wildcards are supported) or specify --all

When the stacks live inside a Stage, this message is unhelpful: their ids are namespaced like StageName/StackName, and neither a bare stack name nor --all is the obvious way to target them, so users are left guessing (see #1451).

This change detects stacks that are nested inside a Stage (i.e. stacks that are not top-level stacks) and additionally points the user at a wildcard pattern that selects them, e.g.:

Some of these stacks are nested inside a Stage. To select the stacks in a Stage, use a pattern that matches their full path, e.g. 'StageName/*'

The hint is only appended when staged stacks are actually present, so flat apps are unaffected. The Stage detection is structural (stacks present recursively but not at the top level) rather than string-based, so a top-level stack whose display name happens to contain / is not misclassified. The same message is shared by the CLI stack-selection path (cloud-assembly.ts) and the toolkit-lib selectStacksV2 path, so both are updated for consistency.

Unit tests were added on both selector/error paths (Stage guidance present when nested, absent for flat apps, and stage patterns de-duplicated across multiple stages).

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.44%. Comparing base (88f6ad1) to head (dd1cea6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1847      +/-   ##
==========================================
+ Coverage   90.31%   90.44%   +0.12%     
==========================================
  Files          80       80              
  Lines       12159    12188      +29     
  Branches     1725     1737      +12     
==========================================
+ Hits        10982    11024      +42     
+ Misses       1143     1130      -13     
  Partials       34       34              

Flag	Coverage Δ
suite.unit	90.44% <100.00%> (+0.12%)	⬆️

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