Add Elastic Agent 8.19.15 release notes (#13858)

* add the 8.19.15 Elastic Agent release notes

* Fix PR links

* Apply suggestions from code review

Co-authored-by: Visha Angelova <91186315+vishaangelova@users.noreply.github.com>

---------

Co-authored-by: pierrehilbert <806950+pierrehilbert@users.noreply.github.com>
Co-authored-by: Pierre HILBERT <pierre.hilbert@elastic.co>
Co-authored-by: Visha Angelova <91186315+vishaangelova@users.noreply.github.com>

Author: 3 people

changelog/8.19.15.asciidoc

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+// begin 8.19.15 relnotes
+
+[[release-notes-8.19.15]]
+==  8.19.15
+
+Review important information about the  8.19.15 release.
+
+[discrete]
+[[security-updates-8.19.15]]
+=== Security updates
+
+
+
+
+* Strip `Unit.Config` from components diagnostic files to prevent secret leakage. https://github.com/elastic/elastic-agent/pull/13338[#13338]
+
+
+
+
+
+
+
+
+
+
+[discrete]
+[[enhancements-8.19.15]]
+=== Enhancements
+
+
+
+
+* Update OTel components to v0.149.0. https://github.com/elastic/elastic-agent/pull/13601[#13601]
+
+
+
+
+[discrete]
+[[bug-fixes-8.19.15]]
+=== Bug fixes
+
+
+
+
+* Persist logging level across agent restarts after policy changes. https://github.com/elastic/elastic-agent/pull/13289[#13289] 
+* Fix graceful shutdown of Beats processes on Windows when Elastic Agent runs as a service. https://github.com/elastic/elastic-agent/pull/13581[#13581]
+* Fix OpenTelemetry collector configuration to correctly merge user-defined extensions with automatically generated ones. https://github.com/elastic/elastic-agent/pull/13639[#13639]
+
+// end 8.19.15 relnotes

changelog/8.19.15.yaml

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+version: 8.19.15
+entries:
+    - kind: bug-fix
+      summary: Persist logging level after agent restart
+      description: |-
+        Persist logging level on policy change, so it is persisted on agent restarts,
+        instead of going back to the default, info level.
+      component: elastic-agent
+      pr:
+        - https://github.com/elastic/elastic-agent/pull/13289
+      issue: []
+      impact: ""
+      action: ""
+      timestamp: 1774526504
+      file:
+        name: 1774526504-persist-logging-level-on-policy-change.yaml
+        checksum: 6ac4295625faede8458f99ba3d9fcc940bbcf18c
+    - kind: security
+      summary: Strip Unit.Config from components diagnostic files to prevent secret leakage
+      description: |
+        The components-expected and components-actual diagnostic files were leaking
+        secrets (API keys, tokens) because Unit.Config contains structpb.Struct fields
+        whose protobuf-style YAML nesting (kind/stringvalue/structvalue) prevented the
+        standard key-based redaction from finding and redacting secret values. The fix
+        strips Unit.Config from the components before serializing to YAML, removing
+        the source of the leak while preserving useful debugging metadata like
+        component IDs, unit IDs, types and log levels.
+      component: elastic-agent
+      pr:
+        - https://github.com/elastic/elastic-agent/pull/13338
+      issue: []
+      impact: ""
+      action: ""
+      timestamp: 1774617287
+      file:
+        name: 1774617287-strip-unit-config-from-components-diagnostics.yaml
+        checksum: efbf3fbe63c0c3c7f1edecc2696dafe5cf9ae04c
+    - kind: bug-fix
+      summary: fix clean stopping of beats on windows
+      description: ""
+      component: elastic-agent
+      pr:
+        - https://github.com/elastic/elastic-agent/pull/13581
+      issue: []
+      impact: ""
+      action: ""
+      timestamp: 1775838969
+      file:
+        name: 1775838969-fix-clean-stopping-of-beats-on-windows.yaml
+        checksum: 7a90f99406b59538dbc3e3b454d4df58e9ff9bfd
+    - kind: enhancement
+      summary: Update OTel components to v0.149.0
+      description: ""
+      component: elastic-agent
+      pr:
+        - https://github.com/elastic/elastic-agent/pull/13601
+      issue: []
+      impact: ""
+      action: ""
+      timestamp: 1776088801
+      file:
+        name: 1776088801-update-otel-components-to-v0.149.0.yaml
+        checksum: df5f7c23492fb858299dc7e884e6401b2ad29301
+    - kind: bug-fix
+      summary: Embedded otel extensions no longer overrides ones generated by inputs.
+      description: ""
+      component: elastic-agent
+      pr:
+        - https://github.com/elastic/elastic-agent/pull/13639
+      issue: []
+      impact: ""
+      action: ""
+      timestamp: 1776251957
+      file:
+        name: 1776251957-fix-otel-embedded-auth.yaml
+        checksum: 2df9f307dab461b5aa9a7a40981536e8a8b0a9f7