Fix some Voice Assistant bugs

[jesserockz]

What does this implement/fix?

Some changes in #6102 had some unexpected side effects when the ring buffer got filled up entirely due to VAD before streaming to HA.

Fix that by allocating 1 more byte in the RingBuffer so the expected number of bytes can be written and read correctly as StreamBuffer will only have space for size - 1

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes esphome/issues#5392

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a8da17) 53.69% compared to head (61a868e) 53.69%.
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #6121   +/-   ##
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  Coverage   53.69%   53.69%           
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  Files          50       50           
  Lines        9398     9398           
  Branches     1652     1652           
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  Hits         5046     5046           
  Misses       4053     4053           
  Partials      299      299           

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