`[Bug]` Fix: Examples are broken and unusable out-of-the-box, preventing user adoption

[Youming-FlowSpec]

Body:

Description

For a new user, the single most important experience is running the examples successfully. Currently, the
examples provided in the /examples directory are broken and fail to run, which creates a significant barrier to
understanding and adopting flowspec-cli.

This issue covers two main problems:

1. The verify command fails because the provided JSON trace files are not in a supported format.
2. The explore command example cannot be run because the required access.log file is missing.

Steps to Reproduce

Problem 1: verify command fails with invalid trace format

1. Clone the repository.
2. Build the project: make build
3. Run the verify command with the provided YAML example:

1 ./build/flowspec-cli verify --path=examples/yaml-contracts/user-service.yaml
--trace=examples/yaml-contracts/test-traces/user-service-trace.json

Expected Behavior:
The command should execute successfully and produce a validation report, demonstrating how the verify command
works.

Actual Behavior:
The command fails with a fatal error:

1 level=error msg="Trace data ingestion failed: failed to detect trace format: unsupported trace format
(detected: unknown). Supported formats: flowspec-trace.json, otlp.json."

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Problem 2: explore command example is missing its data file

1. Examine the examples/nginx-exploration/ directory.
2. Note that the run-example.sh script depends on a file named access.log.
3. Observe that access.log does not exist in the directory or the repository.

Expected Behavior:
The examples directory should contain all necessary files to run the demos, including access.log.

Actual Behavior:
The access.log file is missing, making it impossible to run the explore command example as intended.

Recommendations

1. Update Trace Files: Regenerate all .json trace files in the examples directory to ensure they are in a valid,
   OTel-compatible format that the current version of the CLI can parse.
2. Provide Log File: Add the missing access.log file to the examples/nginx-exploration/ directory so the
   run-example.sh script and the explore command can be tested directly.
3. Verify All Examples: Before closing this issue, confirm that every example script and command mentioned in the
   README.md runs successfully from a fresh clone of the repository.