refactor(lsp): coalesce concurrent server startup in manager

[iceymoss]

Summary

Refactor internal/lsp/manager.go to coalesce concurrent LSP startup requests per server name using singleflight.

Previously, startServer() relied on repeated check-then-create paths. Under concurrent calls, multiple goroutines could still race to create duplicate clients, then close losers. This worked functionally but added unnecessary startup churn and complexity.

Changes

• Add singleflight.Group to lsp.Manager:
  • requestGroup singleflight.Group
• Refactor startServer() into a clearer flow:
  • fast-path checks (unavailable, existing active client),
  • preflight checks (LookPath, skip list, handles),
  • singleflight-protected startup critical section.
• Add helper functions for active client checks:
  • isActiveClient(*Client) bool
  • getActiveClient(name string) (*Client, bool)
• Inside the startup critical section:
  • double-check active client before creating,
  • create/register client once,
  • initialize with timeout,
  • set terminal state (StateReady / StateError),
  • remove failed client from s.clients.
• Use context.WithoutCancel(ctx) for shared startup work so one waiter cancellation does not abort the shared startup operation.
• Reduce duplicate error logs for shared singleflight waiters.

Why

• Avoid redundant client creation under concurrent Start() calls.
• Reduce process churn and improve startup determinism.
• Keep callback/state propagation semantics for app/UI unchanged.

Compatibility

• No public API changes.
• Manager.Start usage remains unchanged.
• Existing server state model and callback behavior are preserved.

Verification

• go test ./internal/lsp
• go test ./internal/lsp/...

Notes

Waiter cancellation now only stops waiting for that caller; the shared startup operation may continue for other callers.

[charmcli]

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[iceymoss]

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[iceymoss]

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