Add useragent attribute for custom LWP::UserAgent configuration

[Copilot]

Adds support for providing a custom LWP::UserAgent instance to XML::Feed for controlling HTTP behavior when fetching feeds from URIs.

Changes

• Constructor: Accepts optional { useragent => $ua } hashref as final parameter with validation

  my $ua = LWP::UserAgent->new;
  $ua->timeout(30);
  my $feed = XML::Feed->new('Atom', { useragent => $ua });

• useragent() method: Getter/setter with lazy initialization

  • Returns existing UserAgent or creates default if none exists
  • Setter validates LWP::UserAgent type

• get_uri(): Uses instance useragent when available, creates default only when needed

• parse(): Calls $feed->get_uri() instead of $class->get_uri() to respect instance useragent

• Validation: Rejects non-LWP::UserAgent objects and unknown hash keys

Maintains backward compatibility - existing code unchanged, RSS version parameter unaffected.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Add "useragent" as a real attribute of the class.</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

@davorg * Add an attribute "useragent" to the class * Accept an optional `\%args` hash to the constructor which contains optional extra attritbutes * Validate the keys of the `\%args` hash - currently only "useragent" is allowed and it must be an object of class "LWP::UserAgent" (or a subclass) * In `get_uri()` only create a UserAgent if one already exists * Add a `useragent` method that returns the UserAgent attribute (creating one if it doesn't exist) * Look into other methods that call `bless` (I see it in `parse()`) and see if they can call `new()` instead

• Fixes Add "useragent" as a real attribute of the class. #67

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[davorg]

@copilot Explain in more depth why you can't replace the bless() in parse() with a call to new()

[Copilot]

@copilot Explain in more depth why you can't replace the bless() in parse() with a call to new()

The bless() in parse() cannot be replaced with new() because of the fundamental difference in how they work:

The Problem:
parse() needs to identify the feed format before creating the properly-typed object, but new() requires the format to be specified upfront.

Flow in parse() (lines 68-101):

1. Line 72: bless {}, $class - Creates temporary XML::Feed object
2. Lines 74-82: Fetch XML content (may call $feed->get_uri(), get_fh(), etc.)
3. Line 93: $feed->identify_format(\$xml) - Determines format by examining the XML
4. Line 99: bless $feed, $format_class - Re-blesses to the correct format class (e.g., XML::Feed::Format::Atom)

Flow in new() (lines 23-64):

1. Line 25: my $format = shift // 'Atom' - Format must be known at construction time
2. Line 57: bless {}, join('::', __PACKAGE__, "Format", $format) - Blesses directly to format class

Why new() won't work:
If we called new() in parse(), we'd need to know the format beforehand, but the whole point of parse() is to auto-detect the format from the XML content. We need the temporary object (line 72) to call instance methods like identify_format() before we know what the final class should be.

The initial bless on line 72 creates a minimal object just functional enough to run format detection, then line 99 re-blesses it to the correct subclass once we know what format it is.

[davorg]

@copilot So can the initial bless() on line 72 be replaced by a call to new()?

[Copilot]

@copilot So can the initial bless() on line 72 be replaced by a call to new()?

No, the initial bless() on line 72 cannot be replaced with new() because:

1. Class mismatch: Line 72 blesses to $class (the base XML::Feed class), but new() always blesses to a format-specific class like XML::Feed::Format::Atom (line 57).

2. new() requires a format: new() expects a format parameter (line 25), but at line 72 we don't know the format yet - that's what we're about to determine.

3. Different purposes: new() calls init_empty() to initialize a new empty feed, but line 72 just needs a minimal object to call identify_format() and handle errors.

The bare bless {}, $class is the correct, minimal approach. To use new() here, we'd need to significantly refactor it to support blessing to the base class and skip initialization - at which point it would essentially be doing the same thing as the current bless.