fix: prevent TypeError in setGitEnvironmentVariables with CDK 2.253+

[hari-euka]

Summary

Fixes #596 — DatadogLambda.addLambdaFunctions() crashes with TypeError: Cannot read properties of undefined (reading 'value') when used with aws-cdk-lib >= 2.253.0.

Root Cause

In src/env.ts, setGitEnvironmentVariables() accesses the internal lam.environment[DD_TAGS].value property unconditionally on line 71, after the else branch calls lam.addEnvironment():

// Before (broken with CDK 2.253+)
lambdas.forEach((lam) => {
    if (lam.environment[DD_TAGS] !== undefined) {
        lam.environment[DD_TAGS].value += `,git.commit.sha:${hash}`;
    } else {
        lam.addEnvironment(DD_TAGS, `git.commit.sha:${hash}`);
    }
    // This line crashes — after addEnvironment(), the internal
    // .environment structure in CDK 2.253 doesn't expose .value
    lam.environment[DD_TAGS].value += `,git.repository_url:${gitRepoUrl}`;
});

In aws-cdk-lib@2.253.0, the internal environment property structure on Lambda functions changed. After calling addEnvironment(), lam.environment[DD_TAGS] may not have a .value property, causing the TypeError.

Fix

Use addEnvironment() via the public API consistently instead of directly mutating the internal .environment property:

// After (works with all CDK versions)
const gitTags = `git.commit.sha:${hash},git.repository_url:${gitRepoUrl}`;

lambdas.forEach((lam) => {
    const existingDdTags = lam.environment[DD_TAGS]?.value;
    if (existingDdTags) {
        lam.addEnvironment(DD_TAGS, `${existingDdTags},${gitTags}`);
    } else {
        lam.addEnvironment(DD_TAGS, gitTags);
    }
});

Key changes:

• Uses optional chaining (?.value) to safely read existing value
• Builds the full git tags string upfront (both git.commit.sha and git.repository_url)
• Uses addEnvironment() for all writes — never directly mutates .value
• Simpler code — no split between setting sha and repo url

Tests

All 240 existing tests pass, including the 4 DD_TAGS tests that verify:

• git.commit.sha and git.repository_url are set by default
• Existing DD_TAGS values are preserved when adding git metadata
• Source code integration can be disabled

Workaround

For users who need an immediate fix before this is released:

const datadog = new DatadogLambda(this, "Datadog", {
    // ... other props
    sourceCodeIntegration: false,
});

[samchungy]

Unfortunately this doesn't actually fix the underlying issue that you can't actually access environment variables anymore: aws/aws-cdk#37773

[TalUsvyatsky]

Thanks for opening this PR, this was ultimately resolved with #601 and https://github.com/DataDog/datadog-cdk-constructs/releases/tag/v2-4.0.0