feat: Improve Mistral and Qwen25 function call parsing

[CatherineSue]

Motivation

This PR focuses to resolve the parallel tool calls parsing for MistralDetector and Qwen25Detector
See Multiple Tool Call Support for MistralDetector and Qwen25Detector for more details.

TL;DR:

• Qwen25Detector's bot_token and eot_token is for single function call. But EBNFComposer was designed for using bot_token and eot_token for the entier function call sequence.
• MistralDetector doesn't support the parallel tool calls parsing yet.

Modifications

refactor EBNF composer API for multiple tool call support

• ebnf_composer.py: Refactor build_ebnf() method to use clearer
  parameter names:
  • Replace bot_token/eot_token with
    sequence_start_token/sequence_end_token for sequence-level wrapping
  • Add individual_call_start_token/individual_call_end_token for
    individual call wrapping
  • Rename TOOL_CALLS_MAP to TOOL_CALL_MAP and update logic to handle
    both sequence and individual call patterns
• qwen25_detector.py: Update to use new individual_call_* parameters
  for per-call token wrapping
• mistral_detector.py: Update parameter names and improve regex comment
  to clarify multiple tool call support
• deepseekv3_detector.py: Update to use new sequence_* parameters
  for sequence-level token wrapping
• pythonic_detector.py: Update to use new sequence_* parameters

improve streaming function call parsing and token handling in Qwen25Detector and MistralDetector

• base_format_detector.py: Add ends_with_partial_token() method to detect
  partial bot tokens during streaming, improving buffer management and preventing
  premature buffer clearing
• qwen25_detector.py: Implement custom streaming parser with buffering to
  handle partial end tokens (</tool_call>) that are streamed character-by-character,
  preventing them from appearing in normal text output
• mistral_detector.py:
  • Refactor JSON parsing to properly handle both single
    objects and arrays, improve error handling with logging, and remove deprecated
    _clean_text() method
  • Improve JSON parsing when there are nested brackets in the arguments, such
    as {"name":"make_next_step_decision", "arguments":{"decision":"","content":"\nTOOL: Access a weather API or service\nOBSERVATION: Retrieve the current weather data for the top 5 populated cities in the US\nANSWER: The weather in the top 5 populated cities in the US is as follows: [City Name] - [Weather Conditions] - [Temperature]\n"}

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
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• Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.

[CatherineSue]

Manual test for chang/tests/examples/test_tool_choice.py

Mistral
mistralai/Mistral-7B-Instruct-v0.3 seems flaky in multiple tool call support.

Qwen25

[MooMoo-Yang]

Does tool call support image return or multi-modal return?