Update REST API header names

[BharatDeva]

PR checklist:

• Did you run ClangFormat ? Not run; this only changes REST API YAML specs.
• Did you separate headers to a different section in existing community code base ? Not applicable.
• Did you surround proton: starts/ends for new code in existing community code base ? Not applicable.

Please write user-readable short description of the changes:

This updates the REST API spec to use the current x-timeplus-* header names instead of the older x-proton-* names.

The server already accepts and emits x-timeplus-* headers in the REST handlers. This keeps the OpenAPI spec aligned with that behavior and cleans up the remaining stale header names under spec/rest-api.

Validation performed:

rg -n x-proton spec/rest-api -g *.yaml

Result: no matches.

git show --check --oneline --summary HEAD

Result:

d8cfc1a5 Update REST API header names

Fixes #728

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