Improve token usage estimate for images

[etraut-openai]

Fixes #11845.

Adjust context/token estimation for inline image data:*;base64,... URLs so we
do not count the raw base64 payload as model-visible text.

What changed:

• keep the existing JSON-length estimator as the baseline
• detect only inline base64 data: image URLs in message and function-call
  output content items
• subtract only the base64 payload bytes (preserving data URL prefix + JSON
  overhead)
• add a fixed per-image estimate of 340 bytes (~85 tokens at the repo’s
  4-bytes/token heuristic)

This avoids large overestimates from MCP image tool outputs while leaving normal
image URLs (https://, file://, non-base64 data: URLs) unchanged.

Tests:

• message image data URL estimate regression
• function-call output image data URL estimate regression
• non-base64 image URLs unchanged
• non-base64 data: URLs unchanged
• data:application/octet-stream;base64,... adjusted
• multiple inline images apply multiple fixed costs
• text-only items unchanged

[etraut-openai]

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