test: complete goto_reply fixture and assertion (#11)

[monkeyxite]

Closes #11

Add fixture tests/mail/fixtures/draft_with_quotes.txt with a realistic reply draft containing >-quoted lines, and rewrite the placeholder test in tests/mail/navigate_spec.lua to load it, call nav.goto_reply(), and assert the cursor lands on a line matching ^>.

Replaces the earlier PR #16 which was auto-marked merged after a branch-pointer mishap during the parallel-agent rollout. This PR contains the actual code changes.

All navigate tests pass (Failed: 0).

[gemini-code-assist]

Code Review

This pull request adds a new test fixture with quoted lines and updates the goto_reply test to verify that the cursor jumps to a quoted line. The feedback suggests improving the test's robustness by asserting the exact line number of the first quote and adding a test case to handle scenarios where no quotes are present.

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[gemini-code-assist]

To make the test suite more robust and ensure correct behavior, we should:

1. Assert the exact line number (8) to verify that goto_reply() indeed jumps to the first quoted line, rather than just any line starting with >.
2. Add a test case to verify that the cursor does not move if there are no quoted lines in the buffer.

      local buf = load_fixture('draft_with_quotes.txt')
      nav.goto_reply()
      local pos = vim.api.nvim_win_get_cursor(0)
      assert.are.equal(8, pos[1])
      local line = vim.api.nvim_buf_get_lines(buf, pos[1] - 1, pos[1], false)[1]
      assert.is_truthy(line:match('^>'))
      vim.api.nvim_buf_delete(buf, { force = true })
    end)

    it('does not move cursor if there are no quotes', function()
      local buf = vim.api.nvim_create_buf(false, true)
      vim.api.nvim_buf_set_lines(buf, 0, -1, false, {
        'From: me@example.com',
        'To: alice@example.com',
        '',
        'Hello Alice,',
        'This is a draft with no quotes.',
      })
      vim.api.nvim_set_current_buf(buf)
      vim.api.nvim_win_set_cursor(0, { 4, 0 })
      nav.goto_reply()
      local pos = vim.api.nvim_win_get_cursor(0)
      assert.are.equal(4, pos[1])
      vim.api.nvim_buf_delete(buf, { force = true })

[monkeyxite]

Addressed in follow-up commit 13d7501:

• Assert cursor lands on line 8 (first > in draft_with_quotes.txt), not just any ^> line.
• New negative test: buffer with no quoted lines → cursor doesn't move.

All 9 navigate_spec tests pass; full suite Failed: 0, Errors: 0.