fix(tools): increase read_file token limit and handle missing model c…

[rishabtiwari-dev]

…ontext length

Description

[ What changed? Feel free to be brief. ]

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Checklist

• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Tests

[ Closes #12432.

• Added a modern baseline context length fallback (64k tokens) if model.contextLength is missing or understated.
• Increased the permissive token limit ratio from 50% to 75% of the model's context length in throwIfFileExceedsHalfOfContext.
• Enhanced the error message to provide actionable guidance on checking configuration or line ranges. ]

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