fix(go): capture local template in Compile closure to prevent sharing

[Zereker]

Summary

• Fix template sharing bug in Compile() method where all compiled PromptFunction closures shared the same dp.Template
• Add regression test TestCompileMultiplePromptsTemplateIsolation to prevent future regressions

Problem

When using the Compile() method multiple times on the same Dotprompt instance, all compiled PromptFunction closures share the same dp.Template field. This causes subsequent prompt executions to use the wrong template.

Root Cause

In dotprompt.go, the Compile method:

1. Creates a new renderTpl from parsing the source
2. Stores it to the shared dp.Template field via initializeTemplate()
3. Returns a closure renderFunc that references dp.Template

When another prompt is compiled, dp.Template gets overwritten, causing all previous renderFunc closures to use the wrong template.

Example

dp := dotprompt.NewDotprompt(nil)

prompt1, _ := dp.Compile("Talk about weather: {{topic}}", nil)
prompt2, _ := dp.Compile("Talk about programming: {{language}}", nil)

// BUG: prompt1 uses prompt2's template!
result1, _ := prompt1(&DataArgument{Input: map[string]any{"topic": "sunny"}}, nil)
// Expected: "Talk about weather: sunny"
// Actual: "Talk about programming: sunny"

Solution

Capture the current template in a local variable before creating the closure:

localTemplate := dp.Template

renderFunc := func(...) {
    renderedString, err := localTemplate.ExecWith(...)  // Use local template
    ...
}

Test Plan

• Added TestCompileMultiplePromptsTemplateIsolation regression test
• All existing tests pass
• Verified fix resolves the issue in real-world usage (Firebase Genkit Go SDK)

Fixes #362

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[Zereker]

I have signed the CLA.

[Zereker]

Test Results

I tested this fix with the memory project using replace directive:

replace github.com/google/dotprompt/go => /tmp/dotprompt/go

Unit test in dotprompt package: ✅ PASS

=== RUN   TestCompileMultiplePromptsTemplateIsolation
--- PASS: TestCompileMultiplePromptsTemplateIsolation (0.00s)

Note: The fix resolves the template sharing issue when compiling multiple different prompts on the same Dotprompt instance. There may be additional caching issues in firebase/genkit when using LookupPrompt with the same .prompt file multiple times, but that would be a separate issue in the genkit repository.