Update aiosignal requirement from >=1.3.0 to >=1.4.0 in /src/a2a

[dependabot]

Updates the requirements on aiosignal to permit the latest version.

Release notes

Sourced from aiosignal's releases.

1.4.0

Features

• Added decorator functionality to Signal as a convenient way to add a callback -- by @Vizonex. [#699](https://github.com/aio-libs/aiosignal/issues/699) <https://github.com/aio-libs/aiosignal/pulls/699>_

• Improved type safety by allowing callback parameters to be type checked (typing-extensions is now required for Python <3.13). Parameters for a Signal callback should now be defined like Signal[int, str] -- by @​Vizonex and @​Dreamsorcerer. [#699](https://github.com/aio-libs/aiosignal/issues/699) <https://github.com/aio-libs/aiosignal/pulls/699>, [#710](https://github.com/aio-libs/aiosignal/issues/710) <https://github.com/aio-libs/aiosignal/pulls/710>

Misc

• Removed the sphinxcontrib-asyncio documentation dependency. [#528](https://github.com/aio-libs/aiosignal/issues/528) <https://github.com/aio-libs/aiosignal/pull/528>_

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Changelog

Sourced from aiosignal's changelog.

1.4.0 (2025-07-03)

Features

• Added decorator functionality to Signal as a convenient way to add a callback -- by @Vizonex. [#699](https://github.com/aio-libs/aiosignal/issues/699) <https://github.com/aio-libs/aiosignal/pulls/699>_

• Improved type safety by allowing callback parameters to be type checked (typing-extensions is now required for Python <3.13). Parameters for a Signal callback should now be defined like Signal[int, str] -- by @​Vizonex and @​Dreamsorcerer. [#699](https://github.com/aio-libs/aiosignal/issues/699) <https://github.com/aio-libs/aiosignal/pulls/699>, [#710](https://github.com/aio-libs/aiosignal/issues/710) <https://github.com/aio-libs/aiosignal/pulls/710>

Misc

• Removed the sphinxcontrib-asyncio documentation dependency. [#528](https://github.com/aio-libs/aiosignal/issues/528) <https://github.com/aio-libs/aiosignal/pull/528>_

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1.3.2 (2024-12-13)

Deprecations and Removals

• Dropped Python 3.7 support. [#413](https://github.com/aio-libs/aiosignal/issues/413) <https://github.com/aio-libs/aiosignal/issues/413>_

• Dropped Python 3.8 support. [#645](https://github.com/aio-libs/aiosignal/issues/645) <https://github.com/aio-libs/aiosignal/issues/645>_

Misc

• [#362](https://github.com/aio-libs/aiosignal/issues/362) <https://github.com/aio-libs/aiosignal/issues/362>_

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1.3.1 (2022-11-08)

Bugfixes

... (truncated)

Commits

• 1cf8014 Fix deploy
• 892494c Release v1.4 (#718)
• fa36082 [pre-commit.ci] pre-commit autoupdate (#719)
• b7f68f1 [pre-commit.ci] pre-commit autoupdate (#717)
• 2b1acac Build(deps): Bump sigstore/gh-action-sigstore-python from 3.0.0 to 3.0.1 (#716)
• 17456ed Build(deps): Bump tox from 4.26.0 to 4.27.0 (#715)
• 4c23690 Build(deps): Bump pytest from 8.4.0 to 8.4.1 (#714)
• 7be2f68 Build(deps): Bump mypy from 1.16.0 to 1.16.1 (#713)
• 5d62945 Build(deps): Bump coverage from 7.9.0 to 7.9.1 (#712)
• a6d85c1 Build(deps): Bump dependabot/fetch-metadata from 2.3.0 to 2.4.0 (#694)
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