Return JSON instead of base64 for EventLog entry attachment download

After this change [1] log entry attachment returns a JSON
instead of a binary blob, it is no longer required to be
base-64 encoded. Return the JSON as-is and modify the
response header to reflect the new format.

Tested:
- Unit tests pass
- Built an image with my code, ran it in QEMU:
1. Injected a log entry with command: [2].
2. Verified LogEntry data [3].
3. Verified Attachment returns JSON data which is no longer base-64
   encoded with expected header [4].
4. Verified Attachment returns 400 with octet-stream header which was
   previously accepted [5].

[1] https://gerrit.openbmc.org/c/openbmc/phosphor-logging/+/89777

[2] Command to inject a log entry and its output:
```
log-create xyz.openbmc_project.Sensor.Threshold.ReadingAboveUpperCriticalThreshold  --json '{ "SENSOR_NAME": "TEST_SENSOR", "READING_VALUE": 99.5, "UNITS": "xyz.openbmc_project.Sensor.Value.Unit.DegreesC", "THRESHOLD_VALUE": 8.0 }'
<3> OPENBMC_MESSAGE_ID={"severity":2,"xyz.openbmc_project.Sensor.Threshold.ReadingAboveUpperCriticalThreshold":{"READING_VALUE":99.5,"SENSOR_NAME":"TEST_SENSOR","THRESHOLD_VALUE":8.0,"UNITS":"xyz.openbmc_project.Sensor.Value.Unit.DegreesC","_SOURCE":{"COLUMN":45,"FILE":"/usr/src/debug/phosphor-logging/1.0+git/log_create_main.cpp","FUNCTION":"int generate_event(const std::string&, const nlohmann::json_abi_v3_12_0::json&, std::optional<int>)","LINE":35,"PID":9445}}}
/xyz/openbmc_project/logging/entry/40
```

[3] Logservices request and output
```
curl -ks -u root:0penBmc -H "Content-Type: application/json"  https://${bmc}/localhost/redfish/v1/Managers/bmc/LogServices/EventLog/Entries/40
{
  "@odata.id": "/redfish/v1/Managers/bmc/LogServices/EventLog/Entries/40",
  "@odata.type": "#LogEntry.v1_9_0.LogEntry",
  "AdditionalDataURI": "/redfish/v1/Managers/bmc/LogServices/EventLog/Entries/40/attachment",
  "Created": "2026-06-23T09:45:05.600-07:00",
  "EntryType": "Event",
  "Id": "40",
  "Message": "Sensor 'TEST_SENSOR' reading of 99.5 (xyz.openbmc_project.Sensor.Value.Unit.DegreesC) is above the 8.0 upper critical threshold.",
  "MessageArgs": [
    "TEST_SENSOR",
    "99.5",
    "xyz.openbmc_project.Sensor.Value.Unit.DegreesC",
    "8.0"
  ],
  "MessageId": "SensorEvent.1.0.ReadingAboveUpperCriticalThreshold",
  "Modified": "2026-06-23T09:45:05.600-07:00",
  "Name": "Manager Event Log Entry",
  "Resolution": "Check the condition of the resources listed in RelatedItem.",
  "Resolved": false,
  "Severity": "Critical"
}
```

[4] LogEntry Attachment
```
curl -ks -u root:0penBmc -H "Accept: application/json" -D  - https://localhost/redfish/v1/Managers/bmc/LogServices/EventLog/Entries/40/attachment
HTTP/1.1 200 OK
Allow: GET
OData-Version: 4.0
Content-Type: application/json
Strict-Transport-Security: max-age=31536000; includeSubdomains
Pragma: no-cache
Cache-Control: no-store, max-age=0
X-Content-Type-Options: nosniff
Date: Tue, 23 Jun 2026 18:07:51 GMT
Content-Length: 814

{
    "additionalData": {
        "READING_VALUE": "99.5",
        "SENSOR_NAME": "TEST_SENSOR",
        "THRESHOLD_VALUE": "8.0",
        "UNITS": "xyz.openbmc_project.Sensor.Value.Unit.DegreesC",
        "_CODE_FILE": "/usr/src/debug/phosphor-logging/1.0+git/log_create_main.cpp",
        "_CODE_FUNC": "int generate_event(const std::string&, const nlohmann::json_abi_v3_12_0::json&, std::optional<int>)",
        "_CODE_LINE": "35",
        "_PID": "9445"
    },
    "associations": [],
    "eventId": "",
    "id": 40,
    "jsonVersion": 1,
    "message": "xyz.openbmc_project.Sensor.Threshold.ReadingAboveUpperCriticalThreshold",
    "resolution": "",
    "resolved": false,
    "severity": 2,
    "timestamp": 1782233105600,
    "updateTimestamp": 1782233105600,
    "version": "catalina-47de2434c3a-dirty"
}
```
[5] Verify that the octet-stream header is rejected
```curl -ks -u root:0penBmc -H "Accept: application/octet-stream" -D  - https://localhost/redfish/v1/Managers/bmc/LogServices/EventLog/Entries/40/attachment
HTTP/1.1 400 Bad Request
Allow: GET
OData-Version: 4.0
Content-Type: application/json
Strict-Transport-Security: max-age=31536000; includeSubdomains
Pragma: no-cache
Cache-Control: no-store, max-age=0
X-Content-Type-Options: nosniff
Date: Tue, 23 Jun 2026 17:03:06 GMT
Content-Length: 814

{
    "additionalData": {
        "READING_VALUE": "99.5",
        "SENSOR_NAME": "TEST_SENSOR",
        "THRESHOLD_VALUE": "8.0",
        "UNITS": "xyz.openbmc_project.Sensor.Value.Unit.DegreesC",
        "_CODE_FILE": "/usr/src/debug/phosphor-logging/1.0+git/log_create_main.cpp",
        "_CODE_FUNC": "int generate_event(const std::string&, const nlohmann::json_abi_v3_12_0::json&, std::optional<int>)",
        "_CODE_LINE": "35",
        "_PID": "9445"
    },
    "associations": [],
    "eventId": "",
    "id": 40,
    "jsonVersion": 1,
    "message": "xyz.openbmc_project.Sensor.Threshold.ReadingAboveUpperCriticalThreshold",
    "resolution": "",
    "resolved": false,
    "severity": 2,
    "timestamp": 1782233105600,
    "updateTimestamp": 1782233105600,
    "version": "catalina-47de2434c3a-dirty"
}
```

Change-Id: Ic6019b80ca6ea32783ae2ffe711adeb5ef2eb157
Signed-off-by: Archit Mehrotra <architmehrotra@meta.com>