test: Correct source maps for test failure reporting

[dlarocque]

When running browser tests with Karma, failed assertions would produce stack traces pointing to line numbers within the final webpack bundle (e.g., chat.test.3891121195.js:26044:55), rather than the source. This made it difficult and time-consuming to trace the failure back to the original source TypeScript file.

Cause:

1. Compilation: .ts compiled to .js, and source maps are generated in .js.map files.
2. Instrumentation: istanbul-instrument-loader injects code into the .js files to generate code coverage reports. Since lines were added to the .js files, the .js.map files are made inaccurate.

Fix:
The previous devtool: 'source-map' config is what instructs webpack to write source maps to a seperate .js.map file. By changing to devtool: 'inline-source-map', the source maps are embedded directly in the JS bundles we load into karma, so the source maps always reflect the state of the .js bundles.

New output:

FAILED TESTS:
  Chat Session
    Google AI gemini-2.0-flash
      ✖ startChat and sendMessage: text input, text output
        Chrome Headless 137.0.0.0 (Mac OS 10.15.7)
      AssertionError: expected 4 to equal 2

      + expected - actual

      -4
      +2

    at Context.<anonymous> (webpack://firebase/ai/integration/chat.test.ts:87:35 <- chat.test.3891121195.js:26044:55)

Old output:

FAILED TESTS:
  Chat Session
    Google AI gemini-2.0-flash
      ✖ startChat and sendMessage: text input, text output
        Chrome Headless 137.0.0.0 (Mac OS 10.15.7)
      AssertionError: expected 4 to equal 2

      + expected - actual

      -4
      +2

    at Context.<anonymous> (chat.test.3891121195.js:26044:55)

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (b97eab3) and merge commit (5600c3d).

Test Logs

• Base (b97eab3): https://github.com/firebase/firebase-js-sdk/actions/runs/15742307895
• Merge (5600c3d): https://github.com/firebase/firebase-js-sdk/actions/runs/15766428408

1. https://storage.googleapis.com/firebase-sdk-metric-reports/9x7MdgRIRX.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (b97eab3) and merge commit (5600c3d).

Test Logs

• Base (b97eab3): https://github.com/firebase/firebase-js-sdk/actions/runs/15742307895
• Merge (5600c3d): https://github.com/firebase/firebase-js-sdk/actions/runs/15766428408

1. https://storage.googleapis.com/firebase-sdk-metric-reports/8xANzYkHVo.html