Add a versioned User-Agent

[Mattsface]

Add a versioned User-Agent

Why

Library-created Sessions currently use Requests' default User-Agent.

A versioned User-Agent makes requests easier to identify during debugging and gives upstream operators better information if the package causes unexpected traffic.

Parent release issue: #265

Required behavior

Library-created Sessions should send a User-Agent similar to:

python-mlb-statsapi/0.9.0

The package version must come from package metadata rather than a separately maintained hardcoded constant.

Session ownership requirements

For a library-created Session:

• Set the package User-Agent
• Preserve other default Session headers
• Use the installed package version

For a caller-injected Session:

• Do not overwrite the caller's User-Agent
• Do not modify other caller-defined headers
• Leave all header configuration under caller control

Applications using an injected Session may continue supplying their own value:

session.headers.update({
    "User-Agent": "my-baseball-project/1.0",
})

Testing requirements

• Library-created Sessions include the package name
• Library-created Sessions include the installed package version
• Package metadata is the version source
• Injected Session User-Agent values are preserved
• Other injected Session headers are preserved
• No network access is required for tests

Suggested branch

feat/versioned-user-agent

The branch should start from and target:

release/0.9.0

Acceptance criteria

• Library-created requests identify the package and version
• Injected Session headers remain untouched
• The version is not duplicated in multiple source files
• Existing Session and retry behavior remains unchanged
• Offline tests pass

Not included

• Machine identifiers
• User tracking
• Telemetry
• Automatic application-name discovery
• Changes to injected Session headers