feat(ast): support moving and copying nodes by path

[SC123667]

What type of PR is this?

feat

Check the PR title.

• This PR title matches the format: (optional scope): .
• The title is user-oriented and clear.
• User-facing AST documentation is updated in this PR.

(Optional) Translate the PR title into Chinese.

feat(ast): 支持按路径移动和复制节点

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

• Add ast.Path, Node.MovePath, and Node.CopyPath for reshaping JSON trees at arbitrary depth.
• Treat an empty path as the root and create missing object/array destination parents from path component types.
• Fill missing array slots with null, preserve existing in-array move/reorder semantics, and reject moves into a source descendant.
• Serialize a copied subtree into a detached lazy node so later source and destination mutations do not alias.
• Keep empty-source-ancestor pruning explicit; this API does not delete unrelated empty parents automatically.
• Document the API and cover the practical reshape/rename scenarios from ast.Node should support move and/or copy at any depth of the tree; not just within arrays #732.

Validation (Go 1.25.12, macOS arm64):

• go test -race across the package set used by the upstream ARM CI workflow.
• (cd issue_test && go test -race ./...).
• SONIC_USE_OPTDEC=1 SONIC_USE_FASTMAP=1 SONIC_ENCODER_USE_VM=1 go test ./ast -count=1.

zh:

• 新增任意深度节点移动/复制 API；支持根路径、自动创建目标父路径、数组补 null、同数组重排与复制隔离。

(Optional) Which issue(s) this PR fixes:

Fixes #732

(optional) The PR that updates user documentation:

The in-repository AST README section is updated in this PR.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve nested destinations when removing an earlier array item

When the source is an array element and the destination is nested beneath a later element of that same array, writing the destination before deleting the source shifts the destination to a different index. For example, moving path {0} to {1, "x"} in ["a", {}] returns success but produces [{"x":"a"}], where the requested destination path no longer exists. The move must account for the index shift or resolve/create the destination after removing the earlier source element.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 64.31373% with 91 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.53%. Comparing base (59be92f) to head (a206fdd).
⚠️ Report is 86 commits behind head on main.

Files with missing lines	Patch %	Lines
ast/node.go	64.31%	90 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #971      +/-   ##
==========================================
+ Coverage   51.86%   53.53%   +1.66%     
==========================================
  Files         127      171      +44     
  Lines       10893    14401    +3508     
==========================================
+ Hits         5650     7710    +2060     
- Misses       4920     6595    +1675     
+ Partials      323       96     -227     

Flag	Coverage Δ
arm	45.83% <64.31%> (?)
macos-15	47.62% <74.12%> (?)
ubuntu-24.04-arm	45.83% <64.31%> (?)
ubuntu-latest	52.42% <74.12%> (?)
x86	52.42% <74.12%> (?)

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