[webgpu] revise implementation of buffer split support

[Yulong Wang (fs-eire)]

Description

This PR addresses a few concerns:

• revert const ProgramBase& -> ProgramBase&: this itself is not doing something wrong but gives much more pressure for who reads the code to understand whether/where the program object is modified. It also can introduce further unexpected modifications to the program object (for example the indirect dispatch code)
• change bool option "ep.webgpuexecutionprovider.smallStorageBufferBindingSizeForTesting" to "ep.webgpuexecutionprovider.maxStorageBufferBindingSize" so now it's possible to set any value in option. (setting to <128MB will cause an assert failure)
• segments are optional in cache key (only present when not equals to 1, which is the common case)
• avoid some unnecessary API calls, which is OK for native but may affect web perf.
• clean up the code a little bit and add a few comments

[Copilot]

Pull Request Overview

This PR refactors the WebGPU provider to replace the testing-only boolean flag small_storage_buffer_binding_size_for_testing with a more flexible max_storage_buffer_binding_size parameter that allows users to explicitly set the maximum storage buffer binding size.

Key changes:

• Replaced boolean testing flag with a configurable uint64_t parameter for controlling storage buffer binding size
• Moved segment calculation from ProgramBase member storage to separate computed vectors
• Made ProgramBase const-correct in Run operations to prevent unintended modifications

Reviewed Changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
webgpu_provider_options.h	Renamed option key from testing-specific to production-ready name
webgpu_provider_factory.cc	Replaced boolean parsing with uint64_t parsing using std::from_chars
webgpu_context.h	Changed parameter type from bool to uint64_t with validation logic
webgpu_context.cc	Updated Run signature to const and moved indirect dispatch handling
shader_helper.h	Added spans for segments and removed FinalizeInputs method
shader_helper.cc	Updated to use segment spans instead of ProgramInput/Output members
program_manager.h	Changed CalculateSegmentsForInputsAndOutputs to return segments via out parameters
program_manager.cc	Implemented segment calculation into separate vectors
program_cache_key.h	Updated signature to accept segment spans
program_cache_key.cc	Updated to use segments from spans with optimization for default values
program.h	Removed segments members from ProgramInput/Output structs and related setters
program.cc	Moved indirect dispatch tensor registration to SetIndirectDispatchTensor
compute_context.h	Updated RunProgram to accept const reference

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