Avoid fallback when definition keys contain expressions

[sibmaks]

Motivation

• Allow definition-key fallback for plain string keys while preserving strict parsing for keys that look like expressions.
• Prevent silent masking of real expression parse errors when a key already contains expression delimiters.

Description

• Added a definitionKeyExpressionFallback option to TemplateCompileOptions to enable opt-in fallback behavior for definition keys.
• Wired TemplateCompilerImpl to pass the new option into RootTemplateExpressionFactory and added compileDefinitionObject to use the fallback-aware compile path.
• Updated RootTemplateExpressionFactory to accept the fallback flag, changed parseObjectKey to accept an allowFallback boolean, and added containsExpression helper to skip fallback when a key contains "{{".
• When fallback is allowed and a key is a plain string, the code now logs a warning and uses a ConstantTemplateExpression for the raw key; expression-like keys will still surface parse errors.

Testing

• No automated tests were executed as part of this change.
• Existing test suite was not run by this rollout.

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Codex Task