Implement log breakpoints

[lionel-]

Branched from #1064.
Addresses posit-dev/positron#12360

Adds support for DAP log breakpoints (logpoints). When a breakpoint has a log message template instead of (or in addition to) a condition, it evaluates the template at hit time, prints the result to the debug console, and does not stop execution. This is the DAP "logpoint" feature.

The log message template uses {expression} placeholders for interpolation, as mandated by the DAP spec. On the R side, interpolation is handled by glue::glue() if we detect a { character. This fails actionnably (message in Console output) if glue is not installed. If there is no { character, the template is returned as is, allowing simple messages to work without the glue dependency.

Log breakpoints respect conditions: if a breakpoint has both a condition and a log message, the condition is evaluated first and the log message only fires when the condition is met. This naturally extends to hit-count breakpoints in the future (see #1087).

Output is emitted through the same emit_breakpoint_output path as conditional breakpoints, using the ```breakpoint fenced block with a clickable file link. When there's no condition, the condition header line (#> ...) is omitted and only the log message content is shown.

With an error we get:

QA Notes

Tested on the backend side.

To test manually:

foo <- function(x) {
  y <- x * 2
  y + 1
}
for (i in 1:5) {
  foo(i)
}

1. Set a log breakpoint on y <- x * 2 with message x is {x}, y will be {x * 2}. Run the loop — should see 5 log messages in the console without stopping.
2. Add a condition x > 3 to the same log breakpoint. Run the loop — should only see messages for x = 4 and x = 5.
3. Try a log message referencing a nonexistent variable like {nonexistent}: should show an inline error without stopping.
4. Without glue installed: a log message without glue syntax like checkpoint reached should work. A log message with glue syntax should fail actionably (error in console output).

[DavisVaughan]

Something seems to not work right in untitled files. It works if I actually save the file. The main annoying thing is that it looks active

Screen.Recording.2026-03-06.at.4.12.29.PM.mov

[lionel-]

That's a general pre-existing issue with breakpoints due to the default way breakpoints look like in the frontend. We don't get breakpoint events with untitiled files.

[DavisVaughan]

That seems much nicer!