fix(c/driver_manager): preserve quoted-key text in profile redefinition errors

[zeroshade]

What

A duplicate quoted key in a connection profile (for example, two
"redshift.db_name" entries) produced a confusing parse error that
doubled the first two characters of the key name:

cannot redefine existing string '"reredshift.db_name" '

instead of naming redshift.db_name.

Root cause

The bug is in the vendored toml++ (v3.4.0). parse_string() advances two
characters to detect a """ multi-line delimiter and appends them to the
parser's diagnostics recording_buffer. When the string turns out to be
single-line, go_back(2u) rewinds the read cursor but does not trim those
two characters from recording_buffer, so the lookahead bytes are duplicated
when a redefinition error renders the key via to_sv(recording_buffer). It
only affects quoted keys (the path through the basic/literal string parser);
bare dotted keys are unaffected.

Fix

Backport the upstream fix into both vendored copies
(c/vendor/toml++/toml.hpp and go/adbc/drivermgr/vendored/toml++/toml.hpp).
The fix snapshots recording_buffer.length() before the lookahead and restores
it after go_back(2u).

• Upstream issue: wrong string in error message marzer/tomlplusplus#300
• Upstream PR: fix(parser): preserve quoted-key text in redefinition error messages marzer/tomlplusplus#302
• Upstream commit: f22f035fe2d63e2be3d266d8f100e7812a9ab9bd

The fix is not in any tagged toml++ release yet (latest is v3.4.0), so it is
backported here. It re-applies cleanly when the vendored copy is next updated.

Testing

Adds ConnectionProfiles.DuplicateQuotedKey to
c/driver_manager/adbc_driver_manager_test.cc, which writes a profile with a
duplicate quoted key and asserts the resulting error names redshift.db_name
and not the doubled reredshift form. Passes locally
(--gtest_filter=ConnectionProfiles.DuplicateQuotedKey).