bug: strategy/system.lua crashes with "Invalid buffer id" in vim.schedule callbacks

[yaadata]

Neovim version: 0.11.6

Operating system/version: macOS 26.3.1

Describe the bug:

The system strategy crashes with "Invalid buffer id" errors when a task's output buffer is closed/wiped while the task is still running or finishing. Two vim.schedule callbacks in strategy/system.lua access self.bufnr without checking buffer validity:

• Line 115 — on_output calls vim.api.nvim_buf_line_count(self.bufnr)
• Line 213 — exit handler sets vim.bo[self.bufnr].modifiable = true

PR #415 (commit 100e969) fixed similar issues in util.lua but these call sites were not covered.

Error executing vim.schedule lua callback: .../overseer/strategy/system.lua:115: Invalid buffer id: 2
stack traceback:
    [C]: in function 'nvim_buf_line_count'
    .../overseer/strategy/system.lua:115: in function ''
    vim/_editor.lua: in function <vim/_editor.lua:0>

Error executing vim.schedule lua callback: .../overseer/strategy/system.lua:213: Invalid buffer id: 3
stack traceback:
    [C]: in function '__newindex'
    .../overseer/strategy/system.lua:213: in function <.../overseer/strategy/system.lua:209>

Severity: tolerable (can work around it)

Steps To Reproduce:

1. Run a task using the system strategy
2. Close/wipe the task output buffer while the task is still running or finishing
3. Errors appear as the vim.schedule callbacks fire on the now-invalid buffer

Expected Behavior:

Callbacks should check vim.api.nvim_buf_is_valid(self.bufnr) and return early if the buffer has been wiped, similar to the guards added in #415.

[yaadata]

I didn't end up using the approach described in the issue that was committed in 100e969

I chose to guard the UI-buffer section up front and let the non-UI behavior continue. That decision was mainly about intent clarity: “buffer is gone, skip UI work, keep task semantics”

yaadata@2a010ce

I don't know if this solution is right and I don't know if this is unnecessary noise so going to publish it here in case it is or if it inspires a different approach