fix: stop pinned context leaking to new chat windows

[laileni-aws]

Problem

Pinned context cannot be un-pinned: after pinning context in a chat, opening a new chat window still shows the pinned item, and un-pinning does not stick.

Root cause

DEFAULT_PINNED_CONTEXT is a shared module-level array (the default pinned context — the active file — used for tabs that have none). In ChatDatabase.addPinnedContext, when a tab had no pinned context yet, the code aliased that shared array and then mutated it:

if (!tab.tabContext.pinnedContext) {
    tab.tabContext.pinnedContext = DEFAULT_PINNED_CONTEXT   // shared reference
}
// ...
tab.tabContext.pinnedContext.unshift(context)  // mutates DEFAULT_PINNED_CONTEXT

So pinning context in one tab permanently added it to the global default. Every new chat window reads DEFAULT_PINNED_CONTEXT for its initial pinned context, so it inherited the leaked item — and un-pinning in the original tab could not remove it from the polluted default. getPinnedContext also returned the shared default array by reference.

Fix

Copy the default array instead of aliasing it:

• addPinnedContext: initialize a tab's pinned context with [...DEFAULT_PINNED_CONTEXT].
• getPinnedContext: return [...DEFAULT_PINNED_CONTEXT] so callers can't mutate the shared default.

Testing

• Added a regression test in chatDb.test.ts asserting that pinning a custom context in a tab does not mutate DEFAULT_PINNED_CONTEXT and that a brand-new tab does not inherit the item.
• The test was confirmed to fail against the previous (aliasing) implementation and pass with this change.
• Ran the chatDb unit tests: 24 passing. prettier --check passes; eslint reports no new issues.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.08%. Comparing base (abb1096) to head (c23b9ab).

Files with missing lines	Patch %	Lines
...language-server/agenticChat/tools/chatDb/chatDb.ts	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2786      +/-   ##
==========================================
- Coverage   60.20%   60.08%   -0.13%     
==========================================
  Files         281      281              
  Lines       71191    71196       +5     
  Branches     4575     4549      -26     
==========================================
- Hits        42861    42776      -85     
- Misses      28241    28331      +90     
  Partials       89       89              

Flag	Coverage Δ
unittests	60.08% <85.71%> (-0.13%)	⬇️

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