Anchor several regex/substring identifier matches against false-positive suffixes

[mkarlesky]

Port of #1273 (green on `next_version`) to `master`, for inclusion in 1.1.8.

Follow-on to #1262 and #1266. Both fixes shared the same root cause — an unanchored match mistaking a substring for a whole token — so after each was fixed, I scanned the rest of the codebase for the same pattern. This PR addresses what that scan found.

Confirmed bugs (empirically reproduced)

`generator_test_results_backtrace.rb` — `do_gdb`'s "prefer whichever unresolved member's own symbol is named in the backtrace" search had no `\b` before the escaped symbol:
```ruby
crash_result[:output].match?( /#{Regexp.escape(tc[:symbol])}\s*()\sat/ )
```
A shorter unresolved symbol that's a suffix of a longer one actually named in the frame (e.g. `foo` vs `my_foo`) could steal the crash attribution. Reproduced with a two-member parameterized group.

`generator_test_runner.rb` — `remap_line_numbers!` matched a test name against source lines completely unescaped and unanchored:
```ruby
if (line =~ /#{next_case[:test]}/)
```
A static helper function whose name merely contains a later test's name as a substring (e.g. `reset_test_ab_state()` before `test_ab`) could false-match first, cascading misalignment to every test case after it in the file. Reproduced the same way.

Defensive hardening

• `c_extractor_declarations.rb`'s `extract_type` replaced `text.rindex(name)` with a `\b`-anchored greedy match (not shown to misfire on realistic declarations, hardened anyway).
• `generator_test_results_backtrace.rb`'s `extract_simple_crash_output` now `Regexp.escape`s an interpolated filename.

Tests

Same coverage as #1273: new regression tests for both confirmed bugs (verified failing pre-fix, passing after), plus a defensive test for the `extract_type` hardening. Unit tests only, no system tests. `bundle exec rake specs:units` clean (1994 examples, 0 failures, 1 pre-existing unrelated pending).

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