[LI (ktz03)]

Motivation & Context

A SendActivity template can rewrite substituted values and leave authored placeholders unresolved. With Local.first = "{Local.second}" and Local.second = "B", {Local.first}|{Local.second} currently produces B|{Local.second} instead of {Local.second}|B. Each replacement searches an already-modified string, so it can consume text inserted by an earlier substitution.

Description & Review Guide

• What are the major changes?

  Replace the sequential string-replacement loop with a regex callback over the original template. Keep the existing matcher, state-path resolution, string conversion, and missing-value behavior. Document that substituted values remain literal.

• What is the impact of these changes?

  Only authored template occurrences are substituted. Repeated and overlapping placeholder values, including references inside lists or dictionaries, remain intact. Thirty-two regression tests cover string and mapping activities, with both ordinary and streaming execution in each test. They change from twelve failures and twenty passing controls to thirty-two passing tests. The complete declarative package suite passes 1,379 tests with no skips and 93% coverage on Windows/Python 3.12.14. Formatting/lint, strict source Pyright, all five test type checkers, applicable prek hooks, and wheel/sdist builds pass. No external service, model, other platform, or performance test was run.

• What do you want reviewers to focus on?

  Preservation of original template positions and literal substituted values, including backslashes, while retaining ordinary interpolation and guarded state-path lookup. Expression evaluation and the expression-result behavior from Python: [BREAKING] preserve expression results in SendActivity output #8509 are unchanged.

Related Issue

Fixes #9095

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused implementation preserves existing behavior while comprehensively testing the reported edge cases.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes declarative template interpolation so substitutions apply only to authored placeholders.

Changes:

• Uses a regex callback over the original template.
• Documents literal substituted values.
• Adds regression coverage for string/mapping and streaming workflows.

File	Description
_declarative_base.py	Corrects interpolation behavior.
test_graph_executors.py	Adds comprehensive regression tests.

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