fix(http12,mcp): set SSE content-type and honor configured MCP message path

[akshatsinha0]

Ensure text/event-stream for SSE and use dubbo.protocol.triple.rest.mcp.path.message in endpoint event
SSE responses use wrong content type; MCP message endpoint ignores config
BY: set text/event-stream, read config

What is the purpose of the change?

Checklist

• [] Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[akshatsinha0]

Just in case, if it is not assigned
@chickenlj
@AlbumenJ
@beiwei30
@CrazyHZM

[akshatsinha0]

@heliang666s
@songxiaosheng

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.17%. Comparing base (aed93d5) to head (0eb72f5).

Files with missing lines	Patch %	Lines
...bo/mcp/transport/DubboMcpSseTransportProvider.java	0.00%	3 Missing ⚠️
...emoting/http12/message/ServerSentEventEncoder.java	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (aed93d5) and HEAD (0eb72f5). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (aed93d5)	HEAD (0eb72f5)
unit-tests-java8	1	0
unit-tests-java11	1	0
unit-tests-java21	1	0
unit-tests-java17	1	0

Additional details and impacted files

@@              Coverage Diff              @@
##                3.3   #15736       +/-   ##
=============================================
- Coverage     60.75%   37.17%   -23.58%     
- Complexity       15    11719    +11704     
=============================================
  Files          1938     1937        -1     
  Lines         88627    88610       -17     
  Branches      13381    13296       -85     
=============================================
- Hits          53842    32939    -20903     
- Misses        29255    51105    +21850     
+ Partials       5530     4566      -964     

Flag	Coverage Δ
integration-tests-java21	32.37% <0.00%> (+0.05%)	⬆️
integration-tests-java8	32.53% <0.00%> (-0.01%)	⬇️
samples-tests-java21	32.12% <0.00%> (+0.06%)	⬆️
samples-tests-java8	29.83% <0.00%> (+0.02%)	⬆️
unit-tests-java11	?
unit-tests-java17	?
unit-tests-java21	?
unit-tests-java8	?

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[RainYuY]

Please take a look the ci problem