Place inline-table separator after the value, not a trailing comment (fix #512)

[gaoflow]

Fixes #512.

Problem

Appending a key to a multiline inline table whose last value is followed by a comment puts the separator comma inside the comment, producing output that no longer round-trips:

doc = tomlkit.loads("tbl = {\n    p = { k = 1 },\n    q = { k = 2 }  # comment\n}\n")
doc["tbl"]["added"] = 3
print(tomlkit.dumps(doc))

tbl = {
    p = { k = 1 },
    q = { k = 2 }  # comment,
added = 3}

The , is now part of the # comment line, so added has no separator and re-parsing the result raises a ParseError.

Cause

In InlineTable.as_string, the separator for a newly-appended key is inserted lazily when the next key is reached:

stripped = buf.rstrip()
buf = f"{stripped},{buf[len(stripped):]}"

buf.rstrip() strips trailing whitespace, but when the previous value is followed by a comment it lands after the comment text rather than after the value, so the comma is swallowed by the comment.

Fix

Track the buffer position right after each rendered value (last_value_end) and splice the deferred separator there. When there is no trailing comment this is exactly where buf.rstrip() pointed, so existing behaviour is unchanged; it only differs in the comment case. Output now round-trips:

tbl = {
    p = { k = 1 },
    q = { k = 2 },  # comment
added = 3}

Tests

Added test_appending_to_inline_table_with_trailing_comment_keeps_comma_outside_comment (asserts the comma is outside the comment and that the result round-trips). It fails on master and passes with the fix; the full suite (993 tests) stays green.