ci: use codspeed `walltime` mode for slow partners (#36049)

Speed up CodSpeed benchmarks for partners with heavy SDK inits by
switching them to walltime mode. `fireworks` takes ~328s and `openai` ~6
min under CPU simulation (Valgrind-based) — walltime is noisier but more
than adequate for detecting init-time regressions on these packages.

## Changes
- Add `CODSPEED_WALLTIME_DIRS` set in `_get_configs_for_single_dir` that
routes `libs/core`, `libs/partners/fireworks`, and
`libs/partners/openai` to walltime mode; all other partners default to
`simulation`
- Emit a `codspeed-mode` field in the CodSpeed matrix config and consume
it as `${{ matrix.job-configs.codspeed-mode }}` in the workflow,
replacing the inline ternary