Fix wrong LinearGradient alignment defaults + allow multiple use of --exclude option in flet build

[ndonkoHenri]

fix #5985
fix #5983
fix #5980

Summary by Sourcery

Fix gradient defaults and improve build CLI exclusion handling.

Bug Fixes:

• Correct the default begin/end alignments for LinearGradient to match expected constants.
• Allow the flet build --exclude option to be specified multiple times to accumulate excluded files and directories.

Enhancements:

• Improve gradient-related docstrings and parameter references for LinearGradient, RadialGradient, and SweepGradient.

Build:

• Update flet build CLI argument definition and help text for the --exclude option.

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine

[Copilot]

Pull request overview

This PR fixes two bugs: incorrect LinearGradient alignment defaults that were calling non-existent methods instead of using class constants, and the inability to use the --exclude option multiple times in flet build commands. Additionally, it includes documentation formatting improvements across gradient classes.

• Fixed LinearGradient.begin and LinearGradient.end to use Alignment.CENTER_LEFT and Alignment.CENTER_RIGHT constants instead of method calls
• Enabled multiple uses of --exclude option in build commands by adding action="extend"
• Improved documentation formatting with better cross-references and code formatting

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sdk/python/packages/flet/src/flet/controls/gradients.py	Fixes LinearGradient alignment defaults from method calls to property access, improves documentation formatting and cross-references
sdk/python/packages/flet-cli/src/flet_cli/commands/build_base.py	Adds action="extend" to --exclude argument to enable multiple uses and updates help text

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