Disambiguate probed configs sharing the same template name in tlBMC

When multiple distinct entity config files share identical unexpanded template names (e.g. "Name": "board_i2c_$bus" or "Card_$index"), tlBMC stored probed configurations in `ProbedConfigMap` keyed by the unexpanded template name string. When subsequent configs with distinct probes (e.g., matching bus 0 vs bus 8) probed true, the latter config's JSON object overwrote earlier matching configs in the map. During `ProcessProbedConfigs`, only the last config's "Exposes" array (and ComputerSystem associations) was applied across all matched FRUs.

Key Changes:
1. Updated `ProbedConfigMap` to `absl::flat_hash_map<std::string, std::vector<ProbedConfigData>>`, storing each probed config's matched FRUs alongside its original JSON configuration.
2. Updated `ProbeFru`, `AddReadyFruConfig`, and `ProcessConfigs` to append and iterate over probed config entries in the map.
3. Updated `ProcessProbedConfigs` to process each `ProbedConfigData` with its corresponding JSON configuration while maintaining continuous `$index` numbering across configs sharing the template name.
4. Added unit test cases in `entity_config_json_impl_test.cc` verifying `$bus` and `$index` substitution when multiple config files share template names.

Google-Bug-Id:548667378,552599743
PiperOrigin-RevId: 972010029
Change-Id: I67cc5c64450595cdfd508fcc5dfa1e388d435911