fix(core): accept non-dict Mapping values in mustache templates

[Lingjin (Haaaarry)]

Fixes #39678

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Mustache templates document their input as a Mapping[str, Any], but only plain dict values actually resolved. Any other Mapping — a ChainMap, UserDict, MappingProxyType, or os.environ — silently rendered as an empty string, so users passing a valid Mapping saw variables disappear with no error and no way to tell a missing key from an unsupported type.

The nested-value lookup now dispatches on the abstract Mapping ABC instead of the concrete dict, so every Mapping resolves through __getitem__ exactly as the API promises. The else branch that rejects traversal into arbitrary objects is unchanged, so the earlier template-injection hardening still applies.

Added a unit test covering ChainMap, UserDict, and MappingProxyType values.

Release note

Mustache prompt templates now resolve variables from any Mapping value (such as ChainMap, UserDict, or os.environ), not only dict.

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This contribution was prepared with AI assistance.

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing Haaaarry:Haaaarry/core/mustache-mapping (01a3729) with master (300eb71)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (9a58107) during the generation of this report, so 300eb71 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩