Improve handling of acceptHeader, setting charset to utf-8

[sheldonrobinson]

1. Changed

final acceptHeader = req.headers.value(HttpHeaders.acceptHeader);
if (acceptHeader == null || !acceptHeader.contains("text/event-stream")) {

to

final acceptHeader = req.headers.value(HttpHeaders.acceptHeader) ?? '';
if (!acceptHeader.contains("text/event-stream")) {

so acceptHeader is never null.

2. Update header content-type to text/event-stream; charset=utf-8 as per MCP Transports Specification

[leehack]

@sheldonrobinson thanks for the contribution!. Would you mind running "dart format ."? It seems the pana is losing 10 points due to the format issue.

[sheldonrobinson]

@leehack Fix format

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.59%. Comparing base (7c53b3b) to head (40f0490).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/server/streamable_https.dart	66.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
- Coverage   78.62%   78.59%   -0.03%     
==========================================
  Files          40       40              
  Lines        5066     5069       +3     
==========================================
+ Hits         3983     3984       +1     
- Misses       1083     1085       +2     

Flag	Coverage Δ
unittests	78.59% <66.66%> (-0.03%)	⬇️

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