docs: improve documentation for predicates and gas price estimation

[airope]

Summary

Closes #3677

Adds documentation for estimatePredicates, estimateGasPrice, and estimatePredicatesAndGasPrice methods to the existing Modifying the Request guide.

Changes

• Added three new sections to modifying-the-request.md:
  • Estimating Gas Price — explains provider.estimateGasPrice() and the blockHorizon parameter
  • Estimating Predicates Gas Usage — explains provider.estimatePredicates() for populating predicateGasUsed on predicate inputs
  • Estimating Predicates and Gas Price Together — explains the combined provider.estimatePredicatesAndGasPrice() method
• Added code snippet file with working examples for all three methods
• References assembleTx guide for the recommended high-level approach

[cursor]

PR Summary

Low Risk
Low risk documentation-only change; main risk is snippet or API usage drifting from actual SDK behavior.

Overview
Adds new guidance to modifying-the-request.md covering Provider.estimateGasPrice, Provider.estimatePredicates, and Provider.estimatePredicatesAndGasPrice, including notes on blockHorizon, predicate predicateGasUsed, and when assembleTx already handles these.

Introduces a new runnable snippet demonstrating funding a predicate, adding predicate resources to a ScriptTransactionRequest, and calling the three estimation methods. Also updates .gitignore to ignore the .claude/ directory.

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