feat(cdk-explorer): add cdk/getConstructTree LSP route

[megha-narayanan]

Exposes the source-decorated construct tree over the language server so a client can overlay source links onto its own tree without re-implementing resolution. The server already builds this tree for hover, definition, and CodeLens; this adds a cdk/getConstructTree request that serializes it.

Each entry is flattened and keyed by construct path (as in tree.json), carrying the resolved source location, the template file, and the character offset of the resource's block in that template.

• No new resolution. The handler serializes the tree the server already caches (cachedIndex), so source locations and template files are exactly what hover and definition use. There is no second resolver to drift.
• Flat entries, not a nested tree. The consumer keeps its own tree and only needs a path to location lookup, so the response is one entry per construct keyed by path.
• templateOffset resolved per request. Each resource's block offset comes from cloud-assembly-api's template ranges, parsing each template at most once per request, so clients get a jump-to-resource position with no template-parsing dependency of their own.
• no-assembly before synth. Until the app is synthesized there is no tree to serve, so the request returns a no-assembly status rather than erroring.

Fixes #

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:

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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 89.64%. Comparing base (58854ba) to head (efaf444).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1746      +/-   ##
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+ Coverage   89.59%   89.64%   +0.04%     
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  Files          78       78              
  Lines       11796    11847      +51     
  Branches     1657     1664       +7     
==========================================
+ Hits        10569    10620      +51     
  Misses       1198     1198              
  Partials       29       29              

Flag	Coverage Δ
suite.unit	89.64% <ø> (+0.04%)	⬆️

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[ShadowCat567]

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