Make text() accessor handle mixed-parts responses

[hsubox76]

response.text() only works when the first Part in the candidate has text, and fails if it's mixed in with FunctionCallParts. Modified to look for text on any Part and added some tests.

We also need to port this fix to GoogleAI.

Internal reference: b/338296397

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[google-oss-bot]

Size Report ¹

Affected Products

• @firebase/vertexai-preview

  Type	Base (4c83b34)	Merge (700023e)	Diff
  browser	25.1 kB	25.2 kB	+85 B (+0.3%)
  main	25.7 kB	25.8 kB	+85 B (+0.3%)
  module	25.1 kB	25.2 kB	+85 B (+0.3%)

• firebase

  Type	Base (4c83b34)	Merge (700023e)	Diff
  firebase-vertexai-preview.js	19.2 kB	19.3 kB	+35 B (+0.2%)

Test Logs

• Base (4c83b34): https://github.com/firebase/firebase-js-sdk/actions/runs/8990674442
• Merge (700023e): https://github.com/firebase/firebase-js-sdk/actions/runs/8992564439

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

[google-oss-bot]

Size Analysis Report ¹

Affected Products

• @firebase/vertexai-preview

  • ChatSession

    Size

    Type	Base (4c83b34)	Merge (700023e)	Diff
    size	14.4 kB	14.4 kB	+42 B (+0.3%)
    size-with-ext-deps	32.8 kB	32.8 kB	+42 B (+0.1%)

  • GenerativeModel

    Size

    Type	Base (4c83b34)	Merge (700023e)	Diff
    size	16.6 kB	16.7 kB	+42 B (+0.3%)
    size-with-ext-deps	35.0 kB	35.1 kB	+42 B (+0.1%)

  • getGenerativeModel

    Size

    Type	Base (4c83b34)	Merge (700023e)	Diff
    size	16.7 kB	16.7 kB	+42 B (+0.3%)
    size-with-ext-deps	35.1 kB	35.2 kB	+42 B (+0.1%)

Test Logs

• Base (4c83b34): https://github.com/firebase/firebase-js-sdk/actions/runs/8990674442
• Merge (700023e): https://github.com/firebase/firebase-js-sdk/actions/runs/8992564439

1. https://storage.googleapis.com/firebase-sdk-metric-reports/3datPDgdbi.html

[dlarocque]

Could we remove the conditional and just return textStrings.join(''), since [].join('') === ''? Or do we prefer to make the return result more explicit

[hsubox76]

Andrew brought up that we're not 100% sure we want the delimiter to be '' when there's multiple text Parts and this is a placeholder for now until we find out, so maybe in the future we might make it a space, or a comma, which could cause some unintended output if we forgot we meant to return '' when there's no Parts.