fix(api): SSE streaming format to comply with specification (#7182)

* Initial plan

* Fix SSE streaming format to comply with specification

- Replace json.Encoder with json.Marshal for explicit formatting
- Use explicit \n\n for all SSE messages (instead of relying on implicit newlines)
- Change %v to %s format specifier for proper string formatting
- Fix error message streaming to include proper SSE format
- Ensure consistency between chat.go and completion.go endpoints

Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

* Add proper error handling for JSON marshal failures in streaming

- Handle json.Marshal errors explicitly in error response paths
- Add fallback simple error message if marshal fails
- Prevents sending 'data: <nil>' on marshal failures
- Addresses code review feedback

Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

* Fix SSE streaming format to comply with specification

Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

* Fix finish_reason field to use pointer for proper null handling

- Change FinishReason from string to *string in Choice schema
- Streaming chunks now omit finish_reason (null) instead of empty string
- Final chunks properly set finish_reason to "stop", "tool_calls", etc.
- Remove empty content from initial streaming chunks (only send role)
- Final streaming chunk sends empty delta with finish_reason
- Addresses OpenAI API compliance issues causing client failures

Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

* Improve code consistency for string pointer creation

- Use consistent pattern: declare variable then take address
- Remove inline anonymous function for better readability
- Addresses code review feedback

Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

* Move common finish reasons to constants

- Create constants.go with FinishReasonStop, FinishReasonToolCalls, FinishReasonFunctionCall
- Replace all string literals with constants in chat.go, completion.go, realtime.go
- Improves code maintainability and prevents typos

Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

* Make it build

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* Fix finish_reason to always be present with null or string value

- Remove omitempty from FinishReason field in Choice struct
- Explicitly set FinishReason to nil for all streaming chunks
- Ensures finish_reason appears as null in JSON for streaming chunks
- Final chunks still properly set finish_reason to "stop", "tool_calls", etc.
- Complies with OpenAI API specification example

Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>

---------

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>
Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>

docs/go.mod

@@ -1,5 +1,3 @@
 module github.com/McShelby/hugo-theme-relearn.git
 
 go 1.19
-
-require github.com/gohugoio/hugo-mod-bootstrap-scss/v5 v5.20300.20200 // indirect

docs/go.sum

@@ -1,4 +0,0 @@
-github.com/gohugoio/hugo-mod-bootstrap-scss/v5 v5.20300.20200 h1:SmpwwN3DNzJWbV+IT8gaFu07ENUFpCvKou5BHYUKuVs=
-github.com/gohugoio/hugo-mod-bootstrap-scss/v5 v5.20300.20200/go.mod h1:kx8MBj9T7SFR8ZClWvKZPmmUxBaltkoXvnWlZZcSnYA=
-github.com/gohugoio/hugo-mod-jslibs-dist/popperjs/v2 v2.21100.20000/go.mod h1:mFberT6ZtcchrsDtfvJM7aAH2bDKLdOnruUHl0hlapI=
-github.com/twbs/bootstrap v5.3.2+incompatible/go.mod h1:fZTSrkpSf0/HkL0IIJzvVspTt1r9zuf7XlZau8kpcY0=