Python: Fix quoted arguments in declarative concatenation

[LI (ktz03)]

Motivation & Context

Declarative string concatenation currently treats any argument beginning and ending with a double quote as a literal. Concatenate("a" & "b", "c") consequently emits a" & "bc instead of abc, and Concatenate("a""b", "c") leaves the escaped quote doubled. This can put formula source into a workflow's rendered activity.

Description & Review Guide

• What are the major changes? Recognize a complete double-quoted literal and decode PowerFx's doubled quotes. Evaluate compound arguments through the existing expression path. The string Concat alias and existing single-quoted compatibility path are preserved.
• What is the impact of these changes? Valid expressions and quoted text render correctly while literal-only concatenation still works without the PowerFx engine. The 38 new regression cases cover factory-created workflows and the engine-unavailable path: 14 fail and 24 pass before the fix, then all 38 pass. The full declarative package passes 1,385 tests with 93% coverage on Windows/Python 3.12.14 with PowerFx 0.0.34. Ruff, source Pyright, all five configured test type checkers, applicable pre-commit hooks, and wheel/sdist builds pass. These are local checks; no external model or deployed service was exercised.
• What do you want reviewers to focus on? Literal recognition, escaped-quote decoding, and preservation of nested expressions, MessageText, missing-value handling, and the engine-independent literal path.

Related Issue

Fixes #9072

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 the title prefix in sync automatically.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused implementation correctly addresses the reported cases with comprehensive regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes declarative concatenation so complete PowerFx string literals are decoded correctly while compound expressions are evaluated normally.

Changes:

• Recognizes complete quoted literals and decodes doubled quotes.
• Adds workflow and engine-unavailable regression coverage for both aliases.

File	Description
_declarative_base.py	Corrects concatenation argument evaluation.
test_graph_workflow_integration.py	Adds workflow-level regression cases.
test_graph_executors.py	Verifies literal concatenation without PowerFx.

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